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Why do meristematic cells have prominent nuclei and dense cytoplasm?


I've been reading about meristematic cells having prominent nuclei and dense cytoplasm. However, I could not understand why it is that way. Could somebody please explain?


Because meristamatic cells are the ones who are undergoing mitosis to increase the height of the plant, and diameter of the plant . They undergo continuous division. So y hey don't mature. They only form the primary cell wall. I wrote another a answer. Perhaps this will answer the question


Meristem

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Meristem, region of cells capable of division and growth in plants. Meristems are classified by their location in the plant as apical (located at root and shoot tips), lateral (in the vascular and cork cambia), and intercalary (at internodes, or stem regions between the places at which leaves attach, and leaf bases, especially of certain monocotyledons—e.g., grasses). Apical meristems give rise to the primary plant body and are responsible for the extension of the roots and shoots. Lateral meristems are known as secondary meristems because they are responsible for secondary growth, or increase in stem girth and thickness. Meristems form anew from other cells in injured tissues and are responsible for wound healing. Unlike most animals, plants continue to grow throughout their entire life span because of the unlimited division of meristematic regions.

Meristematic cells are typically small and nearly spherical. They have a dense cytoplasm and relatively few small vacuoles (watery saclike enclosures). Some of these cells, known as initials, maintain the meristem as a continuing source of new cells and may undergo mitosis (cell division) many times before differentiating into the specific cells required for that region of the plant body. The cells that emanate from the apical meristem are arranged in lineages of partially differentiated tissues known as primary meristems. There are three primary meristems: the protoderm, which will become the epidermis the ground meristem, which will form the ground tissues comprising parenchyma, collenchyma, and sclerenchyma cells and the procambium, which will become the vascular tissues (xylem and phloem).


Multiple Choice Questions

Question 1.
Which of the following tissues has dead cells?
(a) Parenchyma
(b) Sclerenchyma
(c) Collenchyma
(d) Epithelial tissue
Solution:
(b) Sclerenchyma is a simple permanent tissue comprising of dead cells. The cells are long and narrow with lignified cell walls. The cells of sclerenchyma are closely packed without any intercellular spaces. Sclerenchyma tissue is present in stems (around the vascular bundles), roots, veins of leaves, hard covering of seeds and nuts etc. Besides, cells of surface layers in stratified epithelium are dead, but this tissue does not has both dead and living cells unlike sclerenchyma.

Question 2.
Find out incorrect sentence.
(a) Parenchymatous tissues have intercellular spaces.
(b) Collenchymatous tissues are irregularly thickened at corners.
(c) Apical and intercalary meristems are permanent tissues.
(d) Meristematic tissue, in its early stage, lacks vacuoles.
Solution:
(c) Apical and intercalary meristems are not permanent tissues but they are meristematic tissues which remain in continuous state of division. These tissues differentiate to give rise to the permanent tissues.

Question 3.
Girth of stem increases due to
(a) apical meristem
(b) lateral meristem
(c) intercalary meristem
(d) vertical meristem.
Solution:
(b) Lateral meristem occurs on the sides almost parallel to the long axis of the root, stem and its branches. It is responsible for an increase in girth of the stem, i.e. secondary growth.

Question 4.
Which cell does not have perforated cell wall?
(a) Tracheids
(b) Companion cells
(c) Sieve tubes
(d) Vessels
Solution:
(b) Companion cells are narrow, elongated, thin-walled, living cells. They lie on the sides of the sieve tubes and are closely associated with them through plasmodesmata. Companion cells do not possess perforated cell walls.

Question 5.
Intestine absorbs the digested food materials. What type of epithelial cells are responsible for that?
(a) Stratified squamous epithelium
(b) Columnar epithelium
(c) Spindle fibres
(d) Cuboidal epithelium
Solution:
(b) Columnar epithelium consists of pillar-like cells that are much taller than wide. The nuclei are generally elongated along the long axis of cells. It lines the stomach, intestine and gall bladder. Columnar epithelium of intestine is specialised for the absorption of water and digested food.

Question 6.
A person met with an accident in which two long bones of hand were dislocated. Which among the following may be the possible reason?
(a) Tendon break
(b) Break of skeletal muscle
(c) Ligament break
(d) Areolar tissue break
Solution:
(c) It may be ligament break because ligaments serve to bind the bones together. A ligament is an elastic connective tissue which has great strength.

Question 7.
While doing work and running, you move your organs like hands, legs etc. Which among the following is correct?
(a) Smooth muscles contract and pull the ligament to move the bones.
(b) Smooth muscles contract and pull the tendons to move the bones.
(c) Skeletal muscles contract and pull the ligament to move the bones.
(f) Skeletal muscles contract and pull the tendon to move the bones.
Solution:
(d) Skeletal muscles or striated muscles are found in the body wall and the limbs (biceps and triceps of arms, legs). The contraction and relaxation of these muscles are under the control of organism, so they are also called voluntary muscles. These are attached to the bones by tendons. When they contract, they pull the tendon to move the bones and help in the movement of limbs.

Question 8.
Which muscles act involuntarily?
(i) Striated muscles
(ii) Smooth muscles
(iii) Cardiac muscles
(iv) Skeletal muslces
(a) (i) and (ii)
(b) (ii) and (iii)
(c) (iii) and (iv)
(d) (i) and (iv)
Solution:
(b) The muscles which are not under the control of our will, are called involuntary muscles. Smooth (unstriated) muscles and cardiac muscles are involuntary muscles.

Question 9.
Meristematic tissues in plants are
(a) localised and permanent
(b) not limited to certain regions
(c) localised and dividing cells
(d) growing in volume. .
Solution:
(c) Meristematic tissues are groups of living cells which are localised at specific parts in the plant body and divide continuously to add new cells.

Question 10.
Which is not a function of epidermis?
(a) Protection from adverse condition
(b) Gaseous exchange
(c) Conduction of water
(d) Transpiration
Solution:
(c) Epidermis is the outermost layer of cells covering a plant. Its function are principally to protect the plant from injury or adverse conditions and to reduce water loss. It also helps in gaseous exchange and transpiration as it possesses stomata. Conduction of water occurs by xylem which is a complex permanent tissue.

Question 11.
Select the incorrect sentence.
(a) Blood has matrix containing proteins, salts and hormones.
(b) Two bones are connected with ligament.
(c) Tendons are non-fibrous tissue and fragile.
(d) Cartilage is a form of connective tissue.
Solution:
(c) Tendons are white fibrous connective tissues having great strength and join skeletal muscles with bones.

Question 12.
Cartilage is not found in
(a) nose
(b) ear
(c) kidney
(d) larynx.
Solution:
(C) Cartilage is a soft skeletal tissue (a type of connective tissue) which has widely spaced cells. The solid matrix is composed of proteins and sugar. Cartilage smoothens bone surfaces at joints and is also present in the nose, ear, trachea and larynx.

Question 13.
Fats are stored in human body as
(a) cuboidal epithelium
(b) adipose tissue
(c) bones
(d) cartilage.
Solution:
(b) Adipose (connective tissue) is primarily a fat storing tissue in which the matrix is packed with large, spherical or oval fat cells (or adipocytes). Each fat cell contains a large fat globule. The adipose tissue is found beneath the skin, in the covering of the heart, around the blood vessels and kidneys and in yellow bone marrow. This tissue stores fat and insulates the body against heat loss.

Question 14.
Bone matrix is rich in
(a) fluoride and calcium
(b) calcium and phosphorus
(c) calcium and potassium
(d) phosphorus and potassium.
Solution:
(b) Bone is a very strong and non- flexible vertebrate connective tissue which forms the framework that supports the body. The bone matrix contains both inorganic and organic substances. The inorganic salts present in the matrix are mainly phosphates and carbonates of calcium and magnesium e.g. calcium phosphate, calcium carbonate, magnesium phosphate etc.

Question 15.
Contractile proteins are found in
(a) bones
(b) blood
(c) muscles
(d) cartilage.
Solution:
(c) Muscular tissue consists of muscle fibres and is responsible for movement in the body. Muscles contain special proteins called contractile proteins, which contract and relax to cause movement.

Question 16.
Voluntary muscles are found in
(a) alimentary canal
(b) limbs
(c) iris of the eye
(d) bronchi of lungs.
Solution:
(b) Refer to answer 7.

Question 17.
Nervous tissue is not found in
(a) brain
(b) spinal cord
(c) tendons
(d) nerves.
Solution:
(c) The nervous tissue, which contains densely packed nerve cells, called neurons (Gk. neuro = nerve), is present in the brain, spinal cord and nerves. The neurons are specialised for conduction of nerve impulses. Tendon is a type of connective tissue.

Question 18.
Nerve cell does not contain
(a) axon
(b) nerve endings
(c) tendons
(d) dendrites.
Solution:
(c) A nerve cell or neuron comprises of three parts
(i) Cyton or cell body which contains nucleus and cytoplasm with characteristic deeply stained particles called Nissl’s granules.
(ii) Dendrons which are short processes arising from the cyton and further branching into dendrites.
(iii) Axon which is a single, long cylindrical process and forms fine branches terminally (axon endings).

Question 19.
Which of the following helps in repair of tissue and fills up the space inside the organ?
(a) Tendon
(b) Adipose tissue
(c) Areolar
(d) Cartilage
Solution:
(c) Areolar connective tissue is the most widely distributed connective tissue is animal’s body. It is found between the skin and muscles, around blood vessels and nerves, and in the bone marrow. It fills the space inside the organs, supports internal organs and helps in repair of tissues.

Question 20.
The muscular tissue which function through-out the life continuously without fatigue is
(a) skeletal muscle
(b) cardiac muscle
(c) smooth muscle
(d) voluntary muscle.
Solution:
(b) Cardiac muscles are found in the wall of the heart and in the wall of large veins (e.g. pulmonary veins and superior vena cava) where these veins enter the heart. Cardiac muscle fibres are richly supplied with blood vessels and they never get fatigued.

Question 21.
Which of the following cells is found in the cartilaginous tissue of the body?
(a) Mast cells
(b) Basophils
(c) Osteocytes
(d) Chondrocytes
Solution:
(d) Chondrocytes are the cells present in the cartilaginous tissue. These cells produce and maintain the cartilaginous matrix.

Question 22.
The dead element present in the phloem is
(a) companion cells
(b) phloem fibres
(c) phloem parenchyma
(d) sieve tubes.
Solution:
(b) Phloem is a complex permanent tissue of vascular plants which conducts organic food material from leaves to different plant parts. Phloem consists of four types of component viz. sieve tubes, companion cells, phloem parenchyma and phloem fibres. Out of all these components only the phloem fibres are dead and the remaining ones are living.

Question 23.
Which of the following does not lose their nucleus at maturity?
(a) Companion cells
(b) Red blood cells
(c) Vessel
(d) Sieve tube cells
Solution:
(a) Companion cells are narrow, elongated, thin walled, living cells. They have dense cytoplasm and a prominent nucleus. Red blood cells of mammals, vessels (xylem elements) and sieve tube cells (phloem elements) lose their nuclei at maturity.

Question 24.
ln desert plants, rate of water loss gets reduced due to the presence of
(a) cuticle
(b) stomata
(c) lignin
(d) suberin.
Solution:
(a) Cuticle is the continuous waxy layer that covers the aerial parts of a plant. Composed mainly of cutin, it is secreted by the epidermis and its primary function is to prevent water loss.

Question 25.
A long tree has several branches. The tissue that helps in the sideways conduction of water in the branches is
(a) collenchyma
(b) xylem parenchyma
(c) parenchyma
(d) xylem vessels.
Solution:
(b) Xylem (a complex permanent tissue) is the chief conducting tissue of vascular plants responsible for conduction of water and inorganic solutes. The tissue is composed of four kinds of cell: tracheids, vessels (or tracheae), xylem parenchyma and xylem fibres. Vessels and tracheids help in long distance vertical conduction of water and minerals from the roots to different parts of the shoot system. The main stem along with all of its branches are all supplied with water and mineral by mean by xylem vessels and tracheids. Radial conduction i.e., sideways conduction of water occurs by xylem parenchyma.

Question 26.
If the tip of sugarcane plant is removed from the field, even then it keeps on growing in length. It is due to the presence of
(a) cambium
(b) apical meristem
(c) lateral meristem
(d) intercalary meristem.
Solution:
(d) Intercalary meristem is present at the base of intemodes or at the base of nodes and at the base of leaves. This meristem brings about elongation (growth in length) of the part of the plant in which it is present. So even if the tip of the sugarcane plant is removed i.e. apical meristem is removed, it keeps on growing in length due to the activity of intercalary meristem.

Question 27.
A nail is inserted in the trunk of a tree at a height of 1 metre from the ground level. After 3 years the nail will
(a) move downwards
(b) move upwards
(c) remain at the same position
(d) move sideways.
Solution:
(c) The nail will remain at the same position after 3 years, but will appear to have sunk into the tree. This is because of the secondary growth of the tree which increases the girth of a tree. The increase in height of the plant will occur due to primary growth which occurs only in the regions of the apical meristems i.e. at the tips of the main stem or branches. As increase in height of the tree takes place from top only, therefore the nail will remain at the same position as it is inserted near the ground level.

Question 28.
Parenchyma cells are
(a) relatively unspecified and thin walled
(b) thick walled and specialised
(c) lignified
(c) none of these.
Solution:
(a) Parenchyma is a living, simple permanent tissue which consists of relatively unspecified cells with thin walls. The cells contain intercellular spaces. Parenchyma tissue stores food, nutrients, water and performs many other functions.

Question 29.
Flexibility in plants is due to
(a) collenchyma
(b) sclerenchyma
(c) parenchyma
(d) chlorenchyma.
Solution:
(a) Collenchyma is a living, simple permanent tissue which is present below the epidermis in petiole, leaves and stems of herbaceous dicots e.g. sunflower, tomato etc. Collenchyma provides flexibility to the plant parts and allows their easy bending without breaking. Cells of this tissue are elongated, irregularly thickened at the comers with very little intercellular spaces.

Question 30.
Cork cells are made impervious to water and gases by the presence of
(a) cellulose
(b) lipids
(c) suberin
(d) lignin.
Solution:
(c) Cork is the outermost layer of an old woody stem which comprises of dead cells with no intercellular spaces. The cell walls of cork cells are deposited with suberin which makes them impervious to water and gases.

Question 31.
Survival of plants in terrestrial environment has been made possible by the presence of
(a) intercalary meristem
(b) conducting tissue
(c) apical meristem
(d) parenchymatous tissue.
Solution:
(b) Xylem and phloem are the complex permanent tissues which constitute a vascular bundle. Vascular or conducting tissue is a distinctive feature of higher plants, which has made their survival possible in terrestrial environment as .they make it possible for plants to survive without being surrounded with water.

Question 32.
Choose the wrong statement.
(a) The nature of matrix differs according to the function of the tissue.
(b) Fats are stored below the skin and in between the internal organs.
(c) Epithelial tissues have intercellular spaces between them.
(d) Cells of striated muscles are multinudeate and unbranched.
Solution:
(c) Epithelial tissues form the covering or protective tissues in the animal body. The cells of this tissue are tightly packed and form a continuous sheet. They have only a small amount of cementing material between then and almost no intercellular space.

Question 33.
The water conducting tissue generally present in gymnosperm is
(a) vessels
(b) sieve tube
(c) tracheids
(d) xylem fibres.
Solution:
(c) Xylem tissue of gymnosperms lacks the vessels and comprises of tracheids, fibres and parenchyma. Thus, the main water conducting tissue of gymnosperms is xylem tracheid. Sieve tubes translocate food material.

Short Answer Type Questions

Question 34.
Animals of colder regions and fishes of cold water have thicker layer of subcutaneous fat. Describe why?
Solution:
A thick layer of subcutaneous fat functions as an insulating coat that prevents heat loss from the body and helps to keep the body of the animal warm in colder environment. Moreover, fat also functions as reserve food during the periods of scarcity.- Thus, animals of colder regions and fish of cold water have thicker layer of subcutaneous fat.

Question 35.
Match the column (A) with the column (B).Match the column (A) with the column (B).
imageee
Solution:
(a) (v)
(b) (iv)
(c) (iii)
(d) (i)
(e) (ii)

Question 36.
Match the column (A) with the column (B).Match the column (A) with the column (B).
imageee
(a) (i)
(b) (ii)
(c) (iv)
(d) (iii)
(e) (v)

Question 37.
If a potted plant is covered with a glass jar, water vapours appear on the wall of glass jar. Explain why?
Solution:
In a potted plant, transpiration occurs through the stomata present on the surface of leaves. When a potted plant is covered with a glass jar, water vapours released by transpiration condense and appear as water droplets on inner walls of glass jar.

Question 38.
Name the different components of xylem and draw a living component.
Solution:
Different components of xylem are tracheids, vessels, xylem parenchyma and xylem fibres. Xylem parenchyma is the only living component of xylem whose diagram is drawn below:

Question 39.
Draw and identify different elements of phloem.
Solution:
Different elements of phloem are sieve tubes, companion cells, phloem fibres and phloem parenchyma. Different phloem elements are shown in the following diagram:

Question 40.
Write true (T) or false (F).
(a) Epithelial tissue is protective tissue in animal body.
(b) The lining of blood vessels, lung alveoli and kidney tubules are all made up of epithelial tissue.
(c) Epithelial cells have a lot of intercellular spaces.
(d) Epithelial layer is permeable layer.
(e) Epithelial layer does not allow regulation of materials between body and external environment.
Solution:
(a) T
(b) T
(c) F – Epithelial cells have almost no intercellular spaces.
(d) T
(e) F – Due to its permeability, epithelial layer plays an important role in regulating exchange of materials between body and external environment.

Question 41.
Differentiate between voluntary and involuntary muscles. Give one example of each type.
Solution:
Voluntary muscles are the muscles which are under the control of our will e.g. skeletal muscles. Muscles present in our limbs (arms and legs) are skeletal muscles and can be moved by our conscious efforts. not under the control of our will, e.g., smooth muscles and cardiac muscles. Muscles present in stomach, intestine etc. are smooth muscles and muscles present is our heart are cardiac muscles, which cannot be moved by our conscious efforts.

Question 42.
Differentiate the following activities on the basis of voluntary (V) or involuntary (I V) muscles.
(a) Jumping of frog
(b) Pumping of the heart
(c) Writing with hand
(d) Movement of chocolate in your intestine
Solution:
(a) V
(b) IV
(c) V
(d) IV

Question 43.
Fill in the blanks.
(a) Lining of blood vessels is made up of _____.
(b) Lining of small intestine is made up of _____.
(c) Lining of kidney tubules is made up of _____.
(d) Epithelial cells with cilia are found in_____ of our body.
Solution:
(a) squamous epithelium
(b) columnar epithelium
(c) cuboidal epithelium
(d) respiratory tract

Question 44.
Water hyacinth floats on water surface. Explain
Solution:
Water hyacinth (Eichhornia crassipes) is a free-floating aquatic plant which possesses aerenchyma in its spongy petiole. Aerenchyma consists of a network of which enclose very large air cavities. These air cavities store gases, making the plant light and help it to float on the surface of water.

Question 45.
Which structure protects the plant body against the invasion of parasites?
Solution:
Epidermis is a layer of parenchymatous cells that forms the outermost covering of plant body. Epidermis consists of compactly arranged cells without any intercellular spaces. On the aerial plant parts, it secretes a thick, waxy, water-resistant layer called cuticle on its outer surface. These features .make the epidermis protective against loss of water, mechanical injury and the invasion of parasites.

Question 46.
Fill in the blanks.
(a) Cork cells possess _____ on their walls that makes them impervious to gases and water.
(b) _____ have tubular cells with perforated walls and are living in nature.
(c) Bone possesses a hard matrix composed of _____and _____.
Solution:
(a) suberin
(b) Sieve tubes
(c) inorganic, organic substances

Question 47.
Why is epidermis important for the plants?
Solution:
Epiderms is important for the plants as it performs the following important functions :

  1. It protects the internal tissues from mechanical injuries.
  2. It acts as a water-resistant layer and checks loss of water by transpiration.
  3. It protects the plant against invasion of parasitic microorganisms.
  4. It bears stomata on leaves and on young stems, which help in exchange of gases during photosynthesis and respiration. Stomata also act as sites of transpiration.
  5. Root hairs arising from the epidermis of roots help in absorption of water and minerals from the soil.

Question 48.
Fill in the blanks.
(a) ______ are forms of complex tissue.
(b) ______ have guard cells.
(c) Cells of cork contain a chemical called ______.
(d) Husk of coconut is made of ______ tissue.
(e) ______ gives flexibility to plants.
(f) ______ and ______ are both conducting tissues.
(g) Xylem transports ______ and ______ from soil.
(h) Phloem transports ______ from ______ to other parts of the plant.
Solution:
(a) Xylem and phloem
(b) Stomata
(c) suberin
(d) sclerenchyma
(e) Collenchyma
(f) Xylem phloem
(g) water minerals
(h) food leaves

Long Answer Type Questions

Question 49.
Differentiate between sclerenchyma and parenchyma tissues. Draw well labelled diagrams.
Solution:
Differences between parenchyma and sclerenchyma are as follows:

Parenchyma Sclerenchyma
(i) Cells are thin walled with cellulosic cell walls. Cella are thick walled with lignified cell walls.
(ii) It is made up of living cells. It is made up of dead cells.
(iii) Cells are usually loosely packed with large intercellular spaces. No intercellular spaces occur between the cells
(iv) It is primarily a storage tissue and stores nutrients and water in stem and roots. It is primarily a mechanical tissue and provides mechanical strength to the plant part It is primarily a storage tissue and stores nutrients and water in stem and roots.
(v) Some cells contain chlorophyll and form chlorenchyma and perform photosynthesis Other cells have large air cavities and form aerenchyma which provides buoyancy to the hydrophytic plants. The cells are long and narrow, make the plant hard and stiff. The tissue is present in the stem around vascular bundles, in veins of leaves and hard covering of seeds and nuts.

Question 50.
Describe the structure and function of different types of epithelial tissues. Draw diagram of each type of epithelial tissue.
Solution:
Types of Epithelial Tissue: Depending upon the shape and function of the constituent cells, epithelial tissues are of following types:

  • Squamous (cells flattened)
  • Columnar (cells tall, columnar or pillar¬like)
  • Cuboidal (cells cube-like)
  • Ciliated (cells with cilia)
  • Glandular (cells secretory in nature)
  • Stratified (cells many layered)

Squamous epithelium: The cells in this epithelium are extremely thin and flat and are arranged edge to edge forming a delicate lining or covering. It forms the lining of cavities of ducts and blood vessels, lines the chambers of the heart, covers the skin, and lining of the mouth. It also lines pharynx, oesophagus, anal canal, vagina and lower part of urethra. It provides protection to the underlying parts against abrasion (mechanical injury) and entry of germs or chemicals. It also helps in excretion, gas exchange and secretion of coelomic fluid.
Columnar epithelium: This epithelium consists of cells which are much longer than broad and look like a column. It forms the lining of stomach and intestines also found in salivary glands in the mouth, sweat glands and oil glands of the skin. It also lines mammary gland ducts and parts of urethra. It helps in protection, absorption and secretion. Columnar epithelium of intestine is specialised for the absorption of water and digested food.
Cuboidal epithelium: Cells are as long as broad and appear cube-like a centrally located nucleus is present. The cuboidal epithelium * lines the small salivary ducts, pancreatic ducts, sweat glands, salivary glands and thyroid glands. It also covers the ovaries and lines the sperm-producing tubules. It helps in protection, secretion, absorption, excretion and gamete formation.
Ciliated epithelium: This epithelium, usually consisting of cuboidal or columnar cells, has numerous, thin, delicate, hair-like projections called cilia arising from the outer free surface of the cells. It is found lining the wind-pipe (trachea), kidney tubules, oviducts (Fallopian tubes) and ventricles of the brain. This epithelium helps in the movement of mucus, urine, eggs, sperms and cerebrospinal fluid in a particular direction.
Glandular epithelium: This epithelium consists of columnar cells modified to secrete chemicals. It lines the glands such as gastric glands, pancreatic lobules, intestinal glands, etc.
Stratified epithelium: This is a compound epithelium in which cells are arranged in many layers one above the other. It is found in places where there is much wear and tear, such as the epidermis of skin, lining of the mouth cavity.

Question 51.
Draw well labelled diagrams of various types of muscles found in human body.
Solution:
The three main types of muscular tissues found in human body are :
(i) Skeletal (striated) muscle tissue
(ii) Smooth (Non striated) muscle tissue
(iii) Cardiac muscle tissue.
The well labelled diagrams of these tissues are as follows:

Question 52.
Give reasons for:
(a) Meristematic cells have a prominent nucleus and dense cytoplasm but they lack vacuole.
(b) Intercellular spaces are absent in sclerenchymatous tissues.
(c) We get a crunchy and granular feeling, when we chew pear fruit.
(d) Branches of a tree move and bend freely in high wind velocity.
(e) It is difficult to pull out the husk of a coconut tree.
Solution:
(a) Meristematic cells have a prominent nucleus and dense cytoplasm because they are metabolically highly active and are in continuous state of division. Meristematic cells lack vacuole because they do not store food material, waste material, sap etc.
(b) Sclerenchyma cells have lignified cell walls which makes them compact and leaves no intercellular spaces.
(c) Pear fruit contains sclerenchymatous stone cells or sclereids which provide gritty texture to the fruit. Thus, when we chew pear fruit, we get a crunchy and granular feeling.
(d) Collenchyma tissue present in the branches of a tree provides flexibility to them and allows their easy bending without breaking. Thus the branches move and bend freely in high wind velocity.
(e) The husk of a coconut is made up of sclerenchymatous fibres which consist of compactly arranged cells with thick lignified cell walls and no intercellular spaces. So, they are tightly joined together. Thus, it is difficult to pull out the husk of a coconut.

Question 53.
List the characteristics of cork. How are they formed? Mention their role.
Solution:
Cork covers the old stems of woody trees. Characteristics of cork are as follows:

  1. Cells of cork are dead at maturity.
  2. These cells are compactly arranged.
  3. Cells do not possess intercellular spaces.
  4. Cells possess a chemical substance suberin in their walls.
  5. They are several layers thick.
  6. Cork is impervious to gases and water.

As plants grow older, a strip of secondary lateral meristem (called cork cambium) develops in the cortical region. It cuts cells towards both outer and inner sides. Gradually, this secondary tissue replaces the epidermal layer of the stem. This forms the several layer thick cork.
Role of cork is as follows:

  1. It protects the internal tissues from mechanical injury and from parasitic attack.
  2. It contains small pores (called lenticels) for gaseous exchange.
  3. It provides mechanical strength.

Question 54.
Why are xylem and phloem called complex tissues? How are they different from one other?
Solution:
Both xylem and phloem consist of more than one type of cells, which coordinate to perform a common function. Therefore, they are called complex tissues. Differences between xylem and phloem are:

Xylem Phloem
(i) Xylem consists of tracheids, vessels, xylem, parenchyma and xylem fibers. Phloem consists of sieve tubes, companion cells, phloem parenchyma and phloem fibres.
(ii) It transports water and minerals vertically from soil to aerial parts of the plant. It transports food from leaves to other parts of the plant and from storage parts to other parts
(iii) Most of the components of xylem except xylem parenchyma are dead cells. Most of the components of phloem except phloem fibres are living cells.

Question 55.
(a) Differentiate between meristematic and permanent tissues in plants.
(b) Define the process of differentiation.
(c) Name any two simple and two complex permanent tissues in plants/
Solution:
(a) Differences between meristematic and permanent tissues are :

Meristematic tissue

Permanent tissue

(b) Differentiation can be defined as the phenomenon of transformation of unspecialised cells into specialised cells by the loss of ability to divide and by taking up permanent shape, size, structure and function.
(c) Simple permanent tissues in plants are parenchyma, collenchyma and sclerenchyma. Complex permanent tissues in plants are xylem and phloem.


Cells

Meristematic Tissue:

All cells in this tissue types are living.

Permanent Tissue:

Cells in this permanent tissues are both dead and living.

Cells shape

Meristematic Tissue:

Cells are small and isodiametric in shape with large lumen.

Permanent Tissue:

Cells are large, with different shapes.

Cytoplasm

Meristematic Tissue:

All cells in this meristematic tissue have a dense cytoplasm.

Permanent Tissue:

The dead cells in the permanent tissue do not have a cytoplasm.

Nucleus

Meristematic Tissue:

All cells in the meristematic tissue have a prominent nucleus.

Permanent Tissue:

The cells in the permanent tissues may have a nucleus or not (the dead cells)

Vacuoles

Meristematic Tissue:

There are no vacuoles in the meristematic cells.

Permanent Tissue:

Cells of the permanent tissues may have vacuoles or not.

Cells Lumen

Meristematic Tissue:

The meristematic cells are with a large lumen.

Permanent Tissue:

In the permanent tissues cells are with a wide or narrow lumen.

Cell Wall

Meristematic Tissue:

The meristematic cells have very thin and cellulosic wall, with no secondary cell wall thickening.

Permanent Tissue:

The cell wall of the permanent tissue can be tin or thick, lignified or cellulosic. Some cells may undergo secondary cell wall thickening.

Meristematic Tissue:

For the meristematic cells are typical the primary pit fields.

Permanent Tissue:

The pits in the permanent tissue can belong to many different advanced types of pits.

Intercellular Spaces

Meristematic Tissue:

There are no intercellular spaces in the meristematic tissue.

Permanent Tissue:

The permanent tissues can be is loosely packed (parenchyma) or compact (sclerenchyma).

Cell Differentiation

Meristematic Tissue:

The cells of the meristematic tissue are not differentiated.

Permanent Tissue:

The cells of the permanent tissue can be both differentiated and undifferentiated.

Cell Division

Meristematic Tissue:

The undifferentiated cells in the meristematic tissue are capable to rapid division.

Permanent Tissue:

In the permanent tissues, the differentiated cells do not have the ability to divide.

Metabolism of Cells

Meristematic Tissue:

The metabolism in the meristematic tissue is with high rates.

Permanent Tissue:

The metabolism in the permanent tissue is absent or with low rates.

Inclusions and Ergastic Substances

Meristematic Tissue:

No inclusions or ergastic substances are present in these tissues.

Permanent Tissue:

There are inclusions and ergastic substances in these tissues.

Food Reserves

Meristematic Tissue:

This tissue does not store food reserves.

Permanent Tissue:

The cells of some permanent tissues can reserve food (starch).

Location

Meristematic Tissue:

The meristematic tissue is located at determined body parts (root, stems, branches).

Permanent Tissue:

The permanent tissues is present in the entire plant.

Function

Meristematic Tissue:

This tissue is responsible in the growth and formation of new organs throughout the life of the plants.

Permanent Tissue:

The function depends on the tissue type. Permanent tissues provide mechanical support and are involved in conduction, photosynthesis, etc.


Classification based on their Position/Location

Cells of the apical meristem are located at the growing points of the plant. As such, they are present at the shoot, roots as well as branches of the plant. In these locations, they contribute to the length of the plant.

During division, cells of the apical meristem produce new meristematic cells that reside in the shoot tip and roots. Some of the new cells, however, differentiate to produce specialized cells that form different tissues of the plant.

Using cell to cell interactions as well as hormones that act as positional cues, cells of the apical meristem are not only capable of specializing to specific functions (thus forming specific tissues) but also settling in certain parts of the plant. Through the positional cues, certain genes are activated or inhibited thus regulating the differentiation pattern.

The apical meristem is divided in to (SAM) shoot apical meristem (cells located at the tip of branches and plant tip) and the (RAM) root apical meristem where cells are located at the tip of each root.

The primary meristem is basal to the shoot apical meristem (SAM) and is composed of cells that are considered to be in their embryonic stage.

These meristematic cells are divided into the following parts:

The differentiation of the protoderm is one of the major events of embryo development. Here, cells of the protodermal cell layer start differentiating following the polarity of the apical meristem and before the meristems form at the opposite ends of the embryo.

This differentiation results in the production of epidermal cells and consequently the epidermis. In the roots, some of the cells elongate (elongation of the cell walls) and form the root hairs. Although the cells formed here have a thin cell wall, they contain cellulose and pectic substances that help protect cells of the roots.

In addition, protoderm cells continually differentiate to produce new epidermal cells of the roots that ensure that new root hairs continue to be formed given that they have a short lifespan (a few days).

In the roots, protoderm plays an important role in the formation of root hairs that are involved in the absorption of nutrients and water in their environment. The epidermis (which is, for the most part, a single cell layer) also covers all organs in the stem of plants thereby acting like a protective layer.


Samacheer Kalvi 9th Science Organization of Tissues Textbook Exercises

I. Choose the correct answer.

Chapter 18 Organization Of Tissues Question 1.
The tissue composed of a living thin-walled polyhedral cell is …………………
(a) Parenchyma
(b) Collenchyma
(c) Sclerenchyma
(d) None of the above
Answer:
(a) Parenchyma

Organisation Of Tissues Class 9 Question 2.
The fibers consists of ……………………
(a) Parenchyma
(b) Sclerenchyma
(c) Collenchyma
(d) None of the above
Answer:
(b) Sclerenchyma

9th Science Organisation Of Tissues Question 3.
Companion cells are closely associated with ……………………..
(a) sieve elements
(b) vessel elements
(c) trichomes
(d) guard cells
Answer:
(a) sieve elements

Organisation Of Tissues 9th Standard Question 4.
Which of the following is a complex tissue?
(a) parenchyma
(b) collenchyma
(c) xylem
(d) Sclerenchyma
Answer:
(c) xylem

Organisation Of Tissues Question 5.
Aerenchyma is found in ………………………
(a) epiphytes
(b) hydrophytes
(c) halophytes
(d) Xerophytes
Answer:
(b) hydrophytes

9th Organization Of Tissues Question 6.
Smooth muscles occur in ……………………..
(a) uterus
(b) artery
(c) vein
(d) All of the above
Answer:
(a) uterus

Organization Of Tissues Question 7.
Nerve cell does not contain ………………………
(a) axon
(b) nerve endings
(c) tendons
(d) dendrites
Answer:
(c) Tendons

II. 9th Standard Organisation Of Tissues Match the Following:

S. No. Column A S. No. Column B
1. Sclereids a Chlorenchyma
2. Chloroplast b Sclerenchyma
3. Simple tissue c Collenchyma
4. Companion cell d Xylem
5. Tracheids e Phloem

  1. b. Sclerenchyma
  2. a. Chlorenchyma
  3. c. Collenchyma
  4. e. Phloem
  5. d. Xylem

III. Organisation Of Tissues 9th Standard Fill in the blanks.

  1. …………… tissues provides mechanical support to organs.
  2. Parenchyma, collenchyma, Sclerenchyma are …………….. type of tissue.
  3. ……….. and …………. are complex tissues.
  4. Epithelial cells with cilia are found in …………….. of our body.
  5. Lining of small intestine is made up of ………………
  1. Permanent
  2. simple
  3. xylem, phloem
  4. trachea of wind-pipe
  5. columnar epithelium

IV. State whether True or false. If false, write the correct statement

  1. Epithelial tissue is protective tissue in animal body – True
  2. Bone and cartilage are two types of areolar connective tissue – False
    Correct statement: Bone and cartilage are two types of supportive connective tissue.
  3. Parenchyma is a simple tissue – True
  4. Phloem is made up of Tracheids – False
    Correct Statement: Phloem is a complex tissue and constitutes: Sieve elements, Companion cells,
  5. Companion cells and Phloem parenchyma.
  6. Vessels are found in collenchyma – False
    Correct Statement: Vessels are found in xylem.

9th Organization Of Tissues Question 1.
What are intercalary meristems? How do they differ from other meristems?
Answer:
Intercalary meristem lies between the region of permanent tissues and is part of primary meristem which is detached due to formation of intermittent permanent tissues. It is found either at the base of leaf e.g. Pinus or at the base of intemodes e.g. grasses.

Organization Of Tissues Question 2.
What is complex tissue? Name the various kinds of complex tissues.
Answer:
Complex tissues are made of more than one type of cells that work together as a unit. Complex tissues consist of parenchyma and sclerenchyma cells. Common examples are xylem and phloem.

Tissues Class 9 Worksheet With Answers Pdf Question 3.
Mention the most abundant muscular tissue found in our body. State its function.
Answer:
Connective tissue is the most abundant and widely distributed tissue. It provides structural framework and gives support to different tissues forming organs.

Tissues Class 9 Pdf Question 4.
What is skeletal connective tissue? How is it helpful in the functioning of our body?
Answer:
The supporting or skeletal connective tissues form the endoskeleton of the vertebrate body. They support the body, protect various organs and help in locomotion. This system is composed of connective tissues including bone, cartilage, tendons, and ligaments.

Organisation Of Tissue Question 5.
Why should gametes be produced by meiosis during sexual reproduction?
Answer:
Meiosis is important as it produces gametes i.e., male or female germ cells. During meiosis a germ cell or gamete divides to make four new sex cells. As a result of fertilization two gamates join together to form an egg or zygote. Therefore only if gametes are produced, fertilization can take place

Samacheer Kalvi Guru 9th Science Question 6.
In which stage of mitosis the chromosomes align in an equatorial plate? How?+
Answer:
The chromosomes align in an equatorial plate during metaphase stage of mitosis. Each chromosome gets attached to a spindle fibre by its centromere which is known as the chromosomal fibre. During metaphase the sister chromatids are pulled back and forth until they align along the equator of the cell called equatorial plane.

Tissue Exercise Class 9 Question 1.
What are the permanent tissues? Describe the different types of simple permanent tissue.
Answer:
Permanent tissues are those in which, growth has stopped either completely or for the time being. At times, they become meristematic partially or wholly. Permanent tissues are of two types namely

Simple Tissues:
Simple tissue are homogeneous-composed of structurally and functionally similar cells eg., Parenchyma, Collenchyma, and Sclerenchyma.
(i) Parenchyma

Parenchyma are simple permanent tissue composed of living cells. Parenchyma cells are thin-walled, oval, rounded or polygonal in shape with well-developed spaces among them. In aquatic plants. Parenchyma possesses intercellular air spaces and is named as Aerenchyma. When exposed to light, parenchyma cells may develop chloroplasts and are known as Chlorenchyma.
Functions: Parenchyma may store water in many succulent and xerophytic plants. It also serves the functions of storage of food reserves, absorption, buoyancy, secretion etc.

(ii) Collenchyma

Collenchyma is a living tissue found beneath the epidermis.
Cells are elongated with unevenly thickened non-lignified walls.
Cells have rectangular oblique or tapering ends and persistent protoplasts. They possess thick primary non-lignified walls.
Functions: They provide mechanical support for growing organs.

(iii) Sclerenchyma

Sclerenchyma consists of thick-walled cells which are often lignified. Sclerenchyma cells do not possess living protoplasts at maturity.
Sclerenchyma cells are grouped into

  1. fibres and
  2. Sclereids.
  • Fibres are elongated sclerenchymatous cells, usually with pointed ends. Their walls are lignified. Fibres are abundantly found in many plants. The average length of fibres is 1 to 3 mm, however in plants like Linum usitatissimum (flax), Cannabis sativa (hemp) and Corchorus capsularis (jute), fibres are extensively longer ranging from 20 mm to 550 mm.
  • Sclereids are widely distributed in plant body. They are usually broad, may occur in single or in groups. Sclereids are isodiametric, with liginified walls. Pits are prominent and seen along the walls. Lumen is filled with wall materials. Sclereids are also common in fruits and seeds.

Tissues Class 9 Worksheet With Answers Question 2.
Write about the elements of Xylem.
Answer:
Xylem is a conducting tissue which conducts water, mineral nutrients upward from root to leaves. Xylem is also meant for mechanical support to the plant body. Xylem is composed of different kinds of elements. They are

(i) Xylem tracheids: They are elongated or tube-like dead cells with hard, thick and lignified walls. Their ends are tapering, blunt or chisel-like. These cells are devoid of protoplast. They have large lumen without any content. Their function is conduction of water and providing mechanical support to the plant.

(ii) Xylem fibers: These cells are elongated, lignified and pointed at both the ends. Xylem fibres help in conduction of water and nutrients from root to the leaf and also provide mechanical support to the plant.

(iii) Xylem vessels: They are long cylindrical, tube like structures with lignified walls and wide central lumen. These cells are dead as these do not have protoplast. They are arranged in longitudinal series in which the partitioned walls (transverse walls) are perforated, and so the entire structure looks-like a water pipe. Their main function is transport of water and minerals from root to leaf, and also to provide mechanical strength.

(iv) Xylem parenchyma: Its cells are living and thin walled. The main function of xylem parenchyma is to store starch and fatty substances.

Question 3.
List out the differences between mitosis and meiosis.
Answer:

Mitosis Meiosis
Occurs in somatic cells Occurs in reproductive cells
Involved in growth and occurs continuously throughout life Involved in gamete formation only during the reproductively active age
Consists of a single division Consists of two divisions
Two diploid daughter cells are formed Four haploid daughter cells are formed
The chromosome number in the daughter cell is similar to the parent cell (2n) The chromosome number in the daughter cell is just half (n) of the parent cell
Identical daughter cells are formed Daughter cells are not similar to the parent cell and are randomly assorted

VII. Higher Order Thinking Skills.

Tissues Class 9 Solutions Question 1.
What is the consequence that occur if all blood platelets are removed from the blood?
Answer:
If platelets are absent, this important defense reaction cannot occur, and protracted bleeding from small wounds (prolonged bleeding time) results.

Question 2.
Which are not true cells in the blood? Why?
Answer:
Platelets are actually not true cells but merely circulating fragments of cells. But even though platelets are merely cell fragments, they contain many structures that are critical to stop bleeding. They contain proteins on their surface that allow then! to stick to breaks in the blood vessel wall and also to stick to each other. They contain granules that can secrete other proteins required for creating a firm plug to seal blood vessel breaks.

Samacheer Kalvi 9th Science Organization of Tissues Additional Questions

I. Short answers questions.

Question 1.
Name the elements present in phloem.
Answer:
Phloem like xylem is a complex tissue and consists of the following elements.

Question 2.
What is histology?
Answer:
The study of tissues is known as histology.

Question 3.
Describe amitosis.
Answer:
It is the simplest mode of cell division and occurs in unicellular animals, aging cells and in foetal membranes. During amitosis, nucleus elongates first, and a constriction appears in it which deepens and divides the nucleus into two, followed by this cytoplasm divides resulting in the formation of two daughter cells.

Question 4.
Give reasons why the meristematic cells have a large nucleus and dense cytoplasm.
Answer:
Meristematic cells are continuously dividing cells so they have a prominent nucleus and dense cytoplasm. But since these cells do not store food or waste material, they lack vacuole.

II. Long answers questions.

  1. squamous epithelium,
  2. cuboidal epithelium,
  3. columnar epithelium,
  4. ciliated epithelium and
  5. glandular epithelium.
  1. Squamous Epithelium is made up of thin, flat cells with prominent nuclei. It forms delicate lining of the buccal cavity, alveoli of lungs, proximal tubule of kidneys, blood vessels and covering of the skin and tongue. It protects the body from mechanical injury, drying and invasion of germs. It also helps in filtration by forming a selectively permeable membrane surface.
  2. Cuboidal Epithelium is composed of single layer of cubical cells. The nucleus is round and lies in the centre. This tissue is present in the thyroid vesicles, salivary glands, sweat glands and exocrine pancreas. It is also found in the intestine and tubular part of the nephron (kidney tubules) as microvilli that increase the absorptive surface area. Their main function is secretion and absorption.
  3. Columnar Epithelium is composed of a single layer of slender, elongated and pillar like cells. Their nuclei are located at the base. It is found lining the stomach, gall bladder, bile duct, small intestine, colon, oviducts and also forms the mucous membrane. They are mainly involved in secretion and absorption.
  4. Ciliated Epithelium Certain columnar cells bear numerous delicate hair-like outgrowths called cilia and are called ciliated epithelium. Their function is to move particles or mucus in a specific direction over the epithelium. It is seen in the trachea of wind-pipe, bronchioles of the respiratory tract, kidney tubules and fallopian tubes of oviducts.
  5. Glandular Epithelium Epithelial cells are often modified to form specialized gland cells that secrete chemical substances at the epithelial surface. Sometimes a portion of the epithelial tissue folds inward to form a multicellular gland, which lines the gastric glands, pancreatic tubules and intestinal glands.

Question 2.
Write a note on the nervous tissue using a diagram.
Answer:

Nervous tissue comprises of the nerve cells or neurons. They are the longest cells of the body. Neurons are the structural and functional units of the nervous tissue. The elongated and slender processes of the neurons are the nerve fibres. Each neuron consists of a cell body or cyton with nucleus and cytoplasm. The dendrons are short and highly branched protoplasmic processes of cyton. The axon is a single, long fibre like process that develops from the cyton and end up with fine terminal branches. They have the ability to receive stimuli from within or outside the body and send signals to different parts of the body. Many nerve fibres are bound together by the connective tissue.

Question 3.
Describe fluid connective tissue.
Answer:
The blood and the lymph are the fluid connective tissues which link different parts of the body. The cells of the connective tissue are loosely spaced and are embedded in an intercellular matrix.
(a) Blood: contains corpuscles which are red blood cells (erythrocytes), white blood cells (leucocytes) and platelets. In this fluid connective tissue, the blood cells move in a fluid matrix called plasma. The plasma contains inorganic salts and organic substances. It is a main circulating fluid that helps in the transport of substances.

  1. Red blood corpuscles (Erythrocytes): The red blood corpuscles are oval-shaped, circular, biconcave disc-like and lack nucleus when mature (mammalian RBC). They contain a respiratory pigment called hemoglobin which is involved in the transport of oxygen to tissues.
  2. White blood corpuscles (Leucocytes): They are larger in size, contain a distinct nucleus and are colorless. They are capable of amoeboid movement and play an important role in body’s defense mechanism. WBC’s are of two types
    • Granulocytes (with granules in the cytoplasm): have irregular shaped nuclei and cytoplasmic granules. They include neutrophils, basophils, and eosinophils.
    • Agranulocytes (without granules in the cytoplasm): lack cytoplasmic granules and include the lymphocytes which have a spherical nucleus and the monocytes which have a large nucleus indented on one side. They engulf or destroy foreign bodies and neutralize their harmful effects.
    • Blood platelets: They are minute, anucleated, fragile fragments of giant bone marrow called megakaryocytes. They play an important role in the blood clotting mechanism.

(b) Lymph: Lymph is a colorless fluid filtered out of the blood capillaries. It consists of plasma and white blood cells. It mainly helps in the exchange of materials between blood and tissue fluids.


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Plant Tissues

The growth of plants occurs only in certain specific regions. This is because the dividing tissue, also known as meristematic tissue, is located only at these points. Depending on the region where they are present, meristematic tissues are classified as apical, lateral and intercalary. New cells produced by meristem are initially like those of meristem itself, but as they grow and mature, their characteristics slowly change and they become differentiated as components of other tissues.

Apical meristem is present at the growing tips of stems and roots and increases the length of the stem and the root. The girth of the stem or root increases due to lateral meristem (cambium). Intercalary meristem is the meristem at the base of the leaves or internodes (on either side of the node) on twigs.

As the cells of this tissue are very active, they have dense cytoplasm, thin cellulose walls and prominent nuclei. They lack vacuoles.


Anatomy of Flowering Plants

Types of meristems
The meristems may be classified on the basis of their mode of origin, position etc.:
(i) According to origin and development : On the basis of origin, meristematic tissues are of three types:
(a) Promeristem or Primordial meristem : The promeristem originates from embryo and therefore, called primordial or embryonic meristem. It is present in the regions where an organ or a part of plant body is initiated. A group of initial cells that lay down the foundation of an organ or a plant part, is called promeristem. This group consists of limited number of cells, which divide repeatedly to give rise to primary meristem. The promeristem gives rise to all other meristems including the primary meristem.
(b) Primary Meristem: A primary meristem originates from promeristem and retains its meristematic activity. It is located in the apices of roots, stems and the leaf primordial, e.g., Apical meristem and Intercalary meristem.
(c) Secondary Meristem: The secondary meristems are so called because they originate from permanent cells. E.g., phellogen or cork cambium, is an important example of secondary meristem. The secondary meristems produce secondary tissues in the plant body and add new cells for effective protection and repair. They occur in mature regions of roots and shoots.

(ii) According to position : On the basis of their position in the plant body meristems are classified into three categories :

Various meristamatic tissue
(a) Apical meristem : This meristem is located at the growing apices of main, as well as lateral shoots and roots. These cells are responsible for linear growth of an organ. Position of apical cells may either be strictly terminal or subterminal.
(b) Intercalary meristem : These are the portions of apical meristems which are separated from the apex during the growth of axis and formation of permanent tissues. (e.g., Mint), base of internode (e.g., stem of many monocots viz., Wheat, Grasses, Pteridophyets like Equisetum). It occur between mature tissues.
(c) Lateral meristem : These meristems occur laterally in the axis, parallel to the sides of stems and roots. The cambium of vascular bundles (Fascicular, interfascicular and extrastelar cambium) and the cork cambium or phellogen belong to this category and are found in dicotyledons and gymnosperms resulting in an increase in their diameter.

Differences between apical and lateral meristems

S.No. Apical meristem Lateral meristem
1. It occurs at the apex of stem, root and their branches. It is found in lateral position parallel to circumference of the organs.
2. It is a primary meristem. It is secondary meristem, except intra-fascicular cambium.
3. Cells divide in different planes. Cells divide in one plane patricianly both on the outer and inner sides.
4. It produces primary tissues. It gives rise to secondary tissues.
5. It brings about growth in length. It causes growth in girth and thickness.

Structure and Organisation of Apical Meristem
(i) Vegetative shoot apex: Shoot apex was first recognized by Wolff (1759). Shoot apex is derived from meristem present in the plumule of embryo and occurs at the tip of stem and its branches as terminal bud. It also occurs in the inactive state in the axils of leaves as lateral buds. The tip of the shoot apex is dome-shaped and from it flanks, at the base of the dome, one or more leaf primordia. This continues throughout the vegetative phase. Many theories have been put forward to explain shoot apex, such as :

(a) Apical Cell theory: This theory was proposed by Nageli (1858). According to this theory, shoot apical meristem consists of single apical cell. This theory is applicable in case of higher algae, bryophytes and in many pteridophytes but not in higher plants (i.e., gymnosperms and angiosperms).

(b) Histogen theory: It was proposed by Hanstein (1870). According to this theory, the shoot apical meristem consists of three distinct meristematic zones or layers (or histogens).

  • Dermatogen: Outermost layer and it forms epidermis and epidermal tissue system.
  • Periblem: It is the middle layer that gives rise to cortex and endodermis.
  • Plerome: Innermost layer forms pith and stele.

(c) Tunica corpus theory: This theory was proposed by Schmidt (1924). According to this theory, the shoot apex consists of two distinct zones.

  • Tunica: It is mostly single layered and forms epidermis. The cells of tunica are smaller than corpus. The tunica shows only anticlinal division and is responsible for surface growth.
  • Corpus: It represents the central core with larger cells. Corpus shows divisions in all planes and it is responsible for volume growth.

(ii) Root apex: A group of initial cells, present at the subterminal region of the growing root tip, which is protected by a root cap is called root apical meristem or root apex. It is embryonic in origin and formed from the radicle part of embryo. However, in adventitious roots it is produced from derivatives of root apex. The root apex differs from shoot apex as it is short and more or less uniform due to complete absence of lateral appendages (leaves and branches) and differentiation of nodes and internodes. According to Hanstein (1870) root apex of most of the dicotyledons also consists of three meristematic zones – plerome, periblem and dermatogen (fourth meristem calyptrogen to form root cap). Regarding the apical organisation of root following theories have been put forward.
(a) Korper -Kappe theory: It was proposed by Schuepp (1917). This theory is comparable with the tunica and corpus theory of shoot apex. Korper means body and Kappe means cap.

(b) Quiescent centre theory: It was proposed by Clowes (1961). According to him, in addition to actively dividing cells, a zone of inactive cells are present in the central part of the root apex, called quiescent centre. The cells in this region have light cytoplasm, small nuclei, lower concentration of DNA, RNA and protein. These cells also contain fewer number of mitochondria, less endoplasmic reticulum and small dictyosomes.

Permanent Tissues
Permanent tissues are made up of mature cells which develops due to division and differentiation in meristematic tissues. The cells of these tissues are either living or dead, thin-walled or thick-walled. Permanent tissues are of three types :

PERMANENT TISSUES

2. SIMPLE TISSUES
Simple tissues are a group of cells which are all alike in origin, form and function. They are further grouped under three categories :
Parenchyma
Parenchyma is most simple and unspecialized tissue which is concerned mainly with the vegetative activities of the plant. The main characteristics of parenchyma cells are :
(a) The cells are thin-walled and soft.

Parenchyma in T.S.
(b) The cells usually are living and possess a distinct nucleus.
(c) The cells contain well-developed intercellular spaces amongst them.
(d) The cytoplasm is vacuolated and cell wall is made up of cellulose.
(e) The shape may be oval, spherical, cylindrical, rectangular or stellate (star shaped) as in leaf petioles of banana, canna and some hydrophytes.
(f) This tissue is generally present in almost all the organs of plants, i.e., roots, stems, leaves, flowers, fruits and seeds.
(g) If they enclose large air spaces they are called as aerenchyma if they develop chlorophyll, they are called as chlorenchyma and if they are elongated cells with tapering ends, they are called as prosenchyma.

Functions: They perform the following functions :

  • Storage of food materials. g., Carrot, Beetroot etc.
  • Chlorenchyma helps in photosynthesis.
  • Aerenchyma helps in floating of the aquatic plants (Hydrophytes) and also help in gaseous exchange during respiration and photosynthesis, g., Hydrilla.
  • In turgid state they give rigidity to the plant organ.
  • In emergency they behave like meristematic cells and help in healing of the various plant injuries.
  • Sometimes they store secretory substances (ergastic substance) such as tannins, resins and gums and they called as idioblasts.

Collenchyma
It is the tissue of primary body. Occurs in layer below epidermis in dicotyledon stem. Present as a continuous layer or in patches.

  • The cells of this tissue contain protoplasm and are living.
  • The cell walls are thickened at the corners and are made up of cellulose, hemicellulose and pectin.
  • They are compactly arranged cells, oval, spherical or polygonal in outline.
  • No intercellular spaces are present.
  • The tissue is extensible and have capacity to expand.
  • These cells assimilate food when they contain chloroplast.
  • They provide mechanical strength to younger part where xylem is less developed, such as young stem, petiole of leaf etc.
  • They are usually absent in monocots stems and in roots.

Differences between parenchyma and collenchyma

S. No. Parenchyma Collenchyma
1. This is found everywhere in the plant body. It is found in hypodermis of dicot stem and at the leaf base.
2. It can be both primary and secondary tissue. It is usually a primary tissue.
3. Cells are isodiametric. Cells are elongated.
4. It provides turgidity to softer tissues. It provides mechanical strength as well as elasticity to soft structures.
5. This can dedifferentiate rapidly to form meristematic cells. Cells dedifferentiate only rarely.

Sclerenchyma
The main features of sclerenchyma are :

  • It consist of thick-walled dead cells.
  • The cells vary in shape, size and origin.
  • They possess hard and extremely thick secondary walls due to uniform deposition of lignin.
  • In the beginning, the cells are living and have protoplasm but due to deposition of impermeable secondary walls they become dead.

Types of sclerenchyma: They are of two types :

(a) Sclerenchymatous fibres :

  • These are greatly elongated and tapering at both the ends.
  • The fully developed fibre cells are always dead. They are polygonal in transverse section and walls are highly lignified.
  • Intercellular spaces are absent and lumen is highly obliterated. The walls show simple and oblique pits.

  • They provide mechanical strength to the plant.
  • Some of the longest fibre yielding plants are Linumusitatissimum (Flax or Alsi), Corchorus, Cannabis, etc.
  • The fibres are present in hypodermis of monocot stem, in pericycle of many dicots, in secondary wood and vascular bundle sheath in monocot stems.

Types of Fibres

  • On the basis of source and economic use, fibres are of following types:
  • Surface fibres: Fibres obtained from seed coat (testa) of cotton and mesocarp of coconut (coir of commerce) are surface fibres. Cotton fibres are of two types, lint and fuzz. The lint is longer and economically useful while the fuzz is shorter and is not useful.
  • Bast fibres: Fibres obtained from phloem and pericycle are bast or bass fibres. They are most exploited economically, e.g., Jute, Flax, Hemp.
  • Xylary or wood fibres: Fibres associated with xylem are called xylary fibres. Two types of xylary fibres are recognized, libriform fibres and fibre tracheids. Libriform fibres are longer, thick walled and with simple pits while fibre tracheids are shorter, less thickened usually with bordered pits.

(b) Stone cells or Sclereids:

  • They are lignified, extremely thick walled so that the lumen of the cells is almost obliterated. Cells may be spherical, oval, cylindrical, T-shaped or even stellate.
  • They are generally found in hard parts of the plant, e.g., endocarp of Walnut and Coconut.
  • They form part of seed coat in some members of leguminosae. The sclereids provide mechanical support and hardness to the soft parts. Sclereids may be :
    Brachysclereids or stone cells : These are small and more or less isodiametric in shape. They occur in the cortex, pith, phloem, and pulp of fruits (e.g., Pyrus ).
    Macrosclereids or rod cells: These are rod-shaped elongated sclereids usually found in the leaves, cortex of stem and outer seed coats. E.g.,Legumes.
  • Osteosclereids: These are bone or barrel-shaped sclereids dilated at their ends. e.g., leaf of Hakea.
  • Astrosclereids or stellate cells : These are star-shaped sclereids with extreme lobes or arms, e.g., leaf of Nymphaea.
  • Trichosclereids or internal hairs : These are hair-like sclereids found in the intercellular spaces in the leaves and stem of some hydrophytes. E.g., Fig leaf

3. COMPLEX TISSUES
A group of more than one type of cells having common origin and working together as a unit, is called complex permanent tissue. The important complex tissues in vascular plants are : xylem and phloem. Both these tissues together are called as vascular tissue.
Xylem
The term xylem was introduced by Nageli (1858). Xylem is a conducting tissue which conducts water and mineral nutrients upwards from the roots to the leaves.

It also provides mechanical strength to plant parts. On the basis of origin xylem is of two types

Types of Xylem

  • Primary xylem : It is derived from procambium during primary growth. It consists of protoxylem and metaxylem.
  • Secondary xylem : It is formed from vascular cambium during secondary growth.

Xylem Cells
Xylem is composed of four types of cells
(a) Tracheids: The tracheids are elongated tube like cells with tapering or rounded or oval ends with hard and lignified walls.

  • All the vascular plants have tracheids in their xylem.
  • The walls are not much thickened.
  • The cells are without protoplast and are dead on maturity. The tracheids of secondary xylem have fewer sides and are more sharply angular than the tracheids of primary xylem.
  • The cell cavity or lumen of a tracheid is large and without any contents. Tracheids possess bordered pits. Maximum bordered pits are formed in gymnospermous tracheids.
  • They also possess various kinds of thickenings, e.g., annular, spiral, scalariform, reticulate or pitted tracheids.
  • The main function of tracheids is to conduct water and minerals from the root to the leaf. They also provide strength and mechanical support to the plant.

(b) Xylem Vessels: Vessels are rows of elongated tube-like cells, placed end to end with their end walls dissolved.

  • Vessels are multicellular with wide lumen.
  • The vessels may be classified into several types according to the thickening developed in their wall. They may be annular, spiral, scalariform, reticulate or pitted.
  • Vessels are absent in pteridophytes and gymnosperms (except Ephedra, Gnetum, Selaginella, Pteridium).
  • In angiosperms (porous wood) vessels are always present (Vessels are absent in family – Winteraceae and Trochodendraceae of Angiosperm i.e., Wintera, Trochodendron).
  • Vessels along with tracheids form the main tissue of xylem vascular bundles of the angiosperms and help in conduction.
  • It also provides mechanical support to the plant.
  • Vessel members are interconnected through perforations in their common wall.

(c) Wood (xylem) parenchyma:

  • These are the living parenchymatous cells, associated with xylem therefore known as wood parenchyma.
  • They serve as storage of reserve food and also help in conduction of water upwards through tracheids and vessels.
  • Their cell wall is cellulosic, radial conduction of water takes place through ray parenchyma.
  • Radial conduction of water takes place through ray parenchyma.

(d) Wood (xylem) fibres:

  • The long, slender, pointed, dead and sclerenchymatous cells found associated with xylem are termed wood fibres.
  • They possess mostly thickened walls and few small pits. These pits are found abundantly in woody dicotyledons.
  • They provide mechanical strength to xylem and various other organs of plant body.

Phloem (blast)
Term “Phloem” was given by Nageli. Its main function is the transport of organic food materials from leaves to stem and roots in a downward direction. On the basis of origin phloem is of two types:
Types of Phloem
(a) Primary phloem: It is formed by procambium during primary growth.

  • It may or may not show differentiation into protophloem (consists of sieve elements and parenchyma) and metaphloem (develop after protophloem and consists of sieve elements, parenchyma and fiber).
  • During the primary growth the protophloem elements are crushed by the surrounding tissues and disappear. This process is known as obliteration
  • Primary phloem consists of sieve elements, parenchyma and fibre.

(b) Secondary Phloem: It is produced during secondary growth by vascular cambium.
It consists of the following elements :

  • Sieve elements
  • Companion cells
  • Phloem parenchyma
  • Phloem fibres or bast fibres

(1) Sieve element
(i) They are long tube-like cells placed end to end, forming a continuous channel in the plant parts.
(ii) Their cell wall is made up of cellulose.
(iii) Their transverse wall is perforated like a normal sieve and hence they are called as sieve tubes.
(iv) Nucleus is not found in these cells.
(v) Each sieve tube has a lining of cytoplasm near its periphery.
(vi) Their main function is to translocate the food material from one part to the other.

(2) Companion cell

(i) They are thin-walled cells which are associated with sieve tubes.
(ii) They are more or less elongated.
(iii) They are connected with the sieve tube through sieve pore.

Phloem tissues

(iv) They contain nucleus and are therefore, living in nature. This nucleus controls functioning of sieve tube.
(v) They are not found in pteridophytes and gymnosperms but are always present in angiosperms.
(vi) Companion cells helps in maintaining pressure gradient in sieve tube.

(3) Phloem parenchyma : The parenchyma associated with the phloem is called phloem parenchyma. The cells are elongated with rounded ends and possess cellulosic cell walls. These cells are living and store food reserves in the form of starch and fats. They are present in pteridophytes and most of dicotyledonous angiosperms. They are absent in monocots.

(4) Phloem or Bast fibres: The sclerenchymatous fibres associated with the phloem are called as phloem fibres. These are also known as bast fibres. The fibres are elongated lignified cells with simple pits. The ends of these cells may be pointed, needle like or blunt. They are non-living cells that provide mechanical support to the organs.

Differences between Xylem and Phloem

SECRETORY TISSUES
These tissue perform special function in plants, e.g., secretion of resins gum, oil and latex.
These tissues are of two types :
(1) Laticiferous tissues
(2) Glandular tissues
Laticiferous tissues
They are made up of thin walled, elongated, branched and multinucleate (coenocytic) structures that contain colourless, milky or yellow coloured juice called latex. These occur irregularly distributed in the mass of parenchymatous cells. latex is contained inside the laticiferous tissue which is of two types :
(i) Latex cell: A laticiferous cell is a very highly branched cell with long slender processes, ramifying in all directions in the ground tissue of the organ.

  • They do not fuse and form a network.
  • Plants having such tissues are called simple or non-articulated laticifers. e.g., Calotropis (Asclepiadaceae) Nerium, Vinca (Apocyanaceae), Euphorbia (Euphorbiaceae), Ficus (Moraceae).

(ii) Latex vessels : They are formed due to fusion of cells and form network like structure in all directions. At maturity, they form a highly ramifying system of channels full of latex inside the organ. Plants having such tissues are called compound or articulated laticifers. e.g., Argemone, Papaver (Papaveraceae), Sonchus (Compositae), Hevea, Manihot (Euphorbiaceae).
Glandular tissue
This is a highly specialized tissue consisting of glands, discharging diverse functions, including secretory and excretory. Glands may be external or internal.
(i) External glands : They generally occur on the epidermis of stem and leaves as glandular hair as in Plumbago and Boerhaavia, stinging hair secrete poisonous substance in Urtica, nectar secreting glands in flowers or leaves, e.g., Rutaceae and Euphorbiaceae. Digestive enzyme secreting glands in insectivorous plants, e.g., Drosera (Sundew), Nepenthes (Pitcher plant).
(ii) Internal glands: These are present internally and are of several types, e.g., oil glands in Citrus and Eucalyptus, resinous ducts in Pinus, mucilage canals in Cycas. Water secreting glands (hydathodes) in Colocasia (present at the tip of leaves), Tropaeoleum (along margin), etc. The glands which secrete essential oil are called osmophores (osmotrophs).

5. HYDATHODES

  • They are the water exuding structures of plants inhabiting the humid tropics.
  • These hydathodes are found in the leaves of many angiosperms and are located on leaf margins (e.g., Tropaeolum) or at the tip (e.g.. Colocasia).
  • The cells of the mesophyll adjacent to the vascular bundle proliferate to give rise to epithem. The cells of the epithem are thin walled, elongated with dense cytoplasm and deficient in chloroplasts. They have a well-developed system of intercellular spaces and are in close contact with terminal tracheary elements.
  • Overlying the epithem are present two guard cells. The water that moves out of the tracheids under conditions of high root pressure and humidity is ultimately discharged through the terminal pore of the hydathode.

6. THE TISSUE SYSTEM
The various types of tissues present in the body of a plant perform different functions. Several tissues may collectively perform the same function. A collection of tissues performing the same general function is known as a “Tissue System”. There are three major tissue systems present in plants.
(1) Epidermal tissue system
(2) Ground or fundamental tissue system
(3) Vascular or conducting tissue system
Epidermal tissue system
The tissues of this system originate from the outermost layer of apical meristem. It forms the outermost covering of various plant organs which remains in direct contact with the environment.
Epidermis: Epidermis is composed of single layer of cells.

  • These cells vary in their shape and size and form a continuous layer interrupted by stomata. In some cases epidermis may be multilayered, e.g., Ficus, Nerium, Peperomia, Begonia etc.
  • The epidermal cells are living, parenchymatous, and compactly arranged without intercellular spaces.
  • Certain epidermal cells of some plants or plant parts are differentiated into variety of cell types:

(a) In aerial roots, the multiple epidermal cells are modified into velamen, which absorb water from the atmosphere, e. g., Orchids.
(b) Some of the cells in the leaves of grasses are comparatively very large, called bulliform or motor cells. They are hygroscopic in nature, thin-walled and contain big central vacuoles filled with water. They play an important role in the folding and unfolding of leaves. These cells develops from modification of epidermal cell and vein.
(c) Some members of Gramineae and Cyperaceae possess two types of epidermal cells : the long cells and the short cells. The short cells may be cork cells or silica cells.

♦ Cuticle and Wax: In aerial parts of the plant epidermis is covered by cuticle. The epidermal cells secrete a waxy substance called cutin, which forms a layer of variable thickness (the cuticle) within and on the outer surface of its all walls. It helps in reducing the loss of water by evaporation. Other substances deposited on the cuticle surface may be oil, resin, silicon and salts (cystoliths are crystals of calcium carbonate or calcium oxalate, e.g., Ficus. Druse and Raphides, e.g., Pistia). Thick cuticle is found in leaves of dry habita plants.

♦ Stomata: Stomata are minute apertures in the epidermis. Each aperture is bounded by two kidney shaped cells, called guard cells. In xerophytes, the stomata are sunken in grooves due to which rate of transpiration is greatly reduced (e.g. Nerium). Usually there is a large air cavity below each aperture, it is called substomatal cavity. Guard cells are surrounded by subsidiary cells or accessory cells which differ morphologically from the other epidermal cells. In monocots, e.g., Doob, Maize guard cells are dumb bell shape.

  • The outer wall of guard cell is thin and the inner wall is thick .
  • Chloroplast is present in guard cell, which helps in stomatal movement.
  • Stomata aperture, guard cells and subsidiary cells together is refered as stomatal aperture.

♦ Trichomes : These are epidermal outgrowths present temporarily or permanently on almost all plant parts. They may be unicellular or multicellular, vary in size and shape in different species. They may be of different types: stellate hair, glandular hair, short glandular hair, urticating hair and stinging hair. The trichomes serve for checking excess loss of water and for protection.

♦ Root hairs : They are enlargements of special epiblema cells called trichoblasts and occurs in a particular zone of young root called root hair zone. They are specialised to absorb water from soil. They also hold soil particles.
Ground or Fundamental tissue system
Ground tissue system includes all the tissues of plant body except epidermal tissue system and vascular tissues. It forms the bulk of plant body. This tissue system mainly originates from ground meristem. The ground tissue constitute the following parts :
(i) Cortex: It lies between epidermis and the pericycle. The cortex is distinct in dicotyledons but not in monocotyledons, where there is no clear demarcation between cortex and pith. It is further differentiated into
(a) Hypodermis: It is collenchymatous in dicot stem and sclerenchymatous in monocot stem. It provides strength.
(b) General cortex: It consists of parenchymatous cells. Its main function is storage of food.
(c) Endodermis (Starch sheath): It is mostly single layered and is made up of parenchymatous, barrel shaped compactly arranged cells.

  • The inner or transverse wall of endodermal cells have Casparian strips which has deposition of suberin.
  • In roots, thick walled endodermal cells are interrupted by thin walled cells just outside the protoxylem patches. These thin walled endodermal cells are called passage cells.
  • Endodermis with characteristic casparian bands is absent in woody dicot stem, monocot stem and leaves of angiosperms.
  • The young stems of angiosperms show a layer with abundant starch deposition. This layer occurs in the position where endodermis would have been situated which is called as starch sheath.
  • Endodermis behave as water tight dam to check the loss of water and air dam to check the entry of air in xylem elements.
  • Endodermis is internal protective tissue.

(ii) Pericycle: It is a single layered or multilayered cylinder of thin-walled or thick-walled cells present between the endodermis and vascular tissues.
♦ In some cases, the pericycle is made up of many layers of sclerenchymatous cells (Cucurbita stem) or in the form of alternating bands of thin-walled and thick-walled cells (Sunflower stem).
♦ In monocot, the pericycle is made up of thin-walled parenchymatous cells which later on gives rise to lateral roots.
♦ In dicot roots the cork cambium originates in the pericycle which results in the formation of periderm.
♦ Pericycle also gives rise to a part of vascular cambium in dicot roots.

(iii) Pith or medulla: It occupies the central part in dicot stem, and monocot roots.

  • It is mostly made up of parenchymatous cells.
  • In dicot roots pith is completely obliterated by the metaxylem elements.
  • In dicot stem, the pith cells between the vascular bundles become radially elongated and known as primary medullary rays or pith rays. They help in lateral translocation.

Vascular tissue system
The central cylinder of the shoot or root surrounded by cortex is called stele.

  • The varying number of vascular bundles formed inside the stele constitute vascular tissue system.
  • Xylem, phloem and cambium are the major parts of the vascular bundle. Vascular bundle may be of following types :

(i) Radial: The xylem and phloem strands alternate with each other separated by parenchymatous cells. such kinds of vascular bundles are called radial and found mainly in roots.
(ii) Conjoint: A vascular bundle having both xylem and phloem together, is called conjoint. Normally the xylem and phloem occur on the same radius. They occur in stems. Such vascular bundles are of two types :
(a) Collateral: A vascular bundle in which the phloem lies towards outerside and xylem towards inner side, is called collateral, e.g., Sunflower.
Collateral bundle having a cambium between xylem and phloem is said to be of the open type, e.g., Dicot stem.
Collateral bundle lacking a cambium between xylem and phloem is said to be of the closed type, e.g., Monocot stem.
(b) Bicollateral: A vascular bundle having the phloem strands on both outer and inner side of xylem, is called bicollateral. e.g., Cucurbita.
(iii) Concentric: A vascular bundle in which one tissue is completely surrounded by the other, is called concentric. The concentric bundles are of two types :
(a) Amphivasal (Leptocentric): The phloem lies in the centre and remains completely surrounded by xylem. e.g., Dracaena, Yucca.

Various types of vascular bundles : (a) radial (b) conjoint closed (c) conjoint open (b) Amphicribal (Hadrocentric): The xylem lies in the centre and remains completely surrounded by phloem. e.g., Ferns.

7. PRIMARY STRUCTURES IN PLANT

Difference between internal structure of root and stem

Description Root Stem
(i) Epidermis or Epiblema Epiblema or piliferous layer without cuticle Epidermis usually with cuticle.
(ii) Hair Unicellular Multicellular.
(iii) Chlorenchyma in cortex Absent Usually present in young stems but absent in old stem.
(iv) Endodermis Very distinct Poorly developed or absent.
(v) Vascular bundle Radial Conjoint, collateral or bicollateral concentric.
(vi) Xylem Exarch (protoxylem towards periphery) Endarch. (protoxylem towards centre)

Origin of Lateral roots: Lateral roots arise endogenously, i.e., from the cells inside the endodermis. They arise from pericycle cells.

8. INTERNAL STRUCTURE OF LEAF

  • Like the root and stem, the leaf consists of three tissue systems, the dermal system, consisting of the upper and lower epidermis, the ground tissue system, the main photosynthetic tissue, which consists of mesophyll, and the vascular system, comprising of veins of various degrees.
  • Common leaves are bifacial and are further of two types, dorsiventral and isobilateral. Unifacial leaves are cylindrical and occur in onion and garlic.
  • The upper as well as the lower surface of the leaf is covered by a uniseriate epidermis. However, in some plants (e.g., Nerium, Ficus, etc.) the epidermis is multiseriate.
  • All epidermal cells of a leaf are alike. The epidermal cells are compactly arranged and their outer walls are usually thickened. The epidermis is covered by a layer of cuticle the thickness of the, which varies considerably and the xerophytic species have a thicker cuticle.
  • In some xerophytic leaves, especially those of grasses, the epidermal cells situated in longitudinal furrows are large with thin flexible walls. These cells are said to be motor cells or bulliform cells, and they help in rolling of leaves in dry weather.
  • A characteristic feature of the leaf epidermis is the presence of numerous small openings, called stomata. They occur either on both sides of the leaf (leaf is said to be amphistomatic), confined to the lower surface of the leaf (leaf is known as hypostomatic) or to the upper surface as in floating leaves of aquatic plants (leaf is called epistomatic). A stoma consists of two highly specialized epidermal cells, known as guard cells, enclosing a space. In some plants (e.g., Nerium), stomata are present in sunken cavities, called stomatal crypts.
  • The bulk of the internal tissue of the leaf, enclosed by the upper and lower epidermis, forms mesophyll. It is composed of thin walled parenchymatous cells containing numerous chloroplasts. The mesophyll is differentiated into palisade and spongy parenchyma in dicot leaves.
  • The spongy parenchyma consists of irregular and loosely arranged cells, enclosing large intercellular spaces. These air spaces are connected with the substomatal chambers and maintain gaseous exchange with the outside through stomata.
  • The palisade parenchyma is composed of more or less cylindrical and elongated cells arranged compactly with their long axis perpendicular to the epidermis.

Vascular system

  • The mid rib in most dicotyledons consists of a single large collateral vascular bundle with an adaxial xylem and abaxial phloem.
    The cells surrounding the vascular bundles in the leaf are mostly morphologically distinct from the mesophyll cells. These cells constitute the bundle sheath. In dicotyledons the vascular bundles are surrounded by thin walled parenchymatous cells that extend in the direction parallel to the veins. In monocotyledons, the vascular bundles are completely or partially surrounded by one or two bundle sheaths, each consisting of a single layer of cells.

Difference between dicot and monocot leaf

Character Dicot leaf Monocot leaf
(i) Type of leaf Dorsiventral (bifacial) Isobilateral.
(ii) Stomata Usually more on lower epidermis. Equal on lower and upper epidermis (amphistomatic).
(iii) Mesophyll Made up of two types of tissues (a) Palisade parenchyma. (b) Spongy parenchyma with large intercellular spaces Only spongy parenchyma is present, which has very small intercellular spaces.
(iv)Bundle sheath Made up of parenchyma. Just above and below the vascular bundle some parenchymatous cells or collenchymatous cells are present (upto epidermis). Made of parenchyma, but just above and below the vascular bundles sclerenchymatous cells (upto epidermis) are found.
(v) Bulliform or motor cells Absent. Present on upper epidermis.
T.S. of leaf : (a) Dicot (b) Monocot

Kranz type anatomy occurs in both monocot and dicot leaves of some tropical and arid area plants.


Ribosomes

Ribosomes are small structures where proteins are made. Although they are not enclosed within a membrane, they are frequently considered organelles. Each ribosome is formed of two subunits, like the one pictured at the top of this section. Both subunits consist of proteins and RNA. RNA from the nucleus carries the genetic code, copied from DNA, which remains in the nucleus. At the ribosome, the genetic code in RNA is used to assemble and join together amino acids to make proteins. Ribosomes can be found alone or in groups within the cytoplasm as well as on the RER.


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