How does P-protein (phloem) help in sealing of wounds?

While I was reading a book I found that "P-proteins in the sieve elements of phloem help in sealing of wounds along with callose".

So how does p-protein do that, moreover how can it reach to other parts of plant body in order to seal the wounds?

Interaction of xylem and phloem during exudation and wound occlusion in Cucurbita maxima

Collection of cucurbit exudates from cut petioles has been a powerful tool for gaining knowledge on phloem sap composition without full notion of the complex exudation mechanism. Only few publications explicitly mentioned that exudates were collected from the basal side of the cut, which exudes more copiously than the apical side. This is surprising since only exudation from the apical side is supposedly driven by phloem pressure gradients. Composition of carbohydrates and pH values at both wounding sides are equal, whereas protein concentration is higher at the basal side. Apparently, exudation is far more complex than just the delivery of phloem sap. Xylem involvement is indicated by lower protein concentrations after elimination of root pressure. Moreover, dye was sucked into xylem vessels owing to relaxation of negative pressure after cutting. The lateral water efflux from the vessels increases turgor of surrounding cells including sieve elements. Simultaneously, detached parietal proteins (PP1/PP2) induce occlusion of sieve plates and cover wound surface. If root pressure is strong enough, pure xylem sap can be collected after removal of the occlusion plug at the wound surface. The present findings provide a mechanism of sap exudation in Cucurbita maxima, in which the contribution of xylem water is integrated.

Plant Cell Biology

Xu Wang , . Jung-Youn Lee , in Methods in Cell Biology , 2020

2.4 Callose staining

Callose staining does not in itself report plasmodesmal permeability but is a useful complementary method to inform if changes in plasmodesmal permeability are linked to alterations in callose accumulation levels. Callose levels at plasmodesmata can be detected using the stain aniline blue, or by using an immuno-fluorescence or -gold labeling approach, then quantified using confocal or transmission electron microscopy. Aniline blue-based callose measurement can be done in both live and fixed tissues, and the procedure is relatively simple. However, the dye is highly prone to photobleaching, hence imaging needs to be done with extra caution.

Cambium: Origin, Duration and Function (With Diagrams) | Botany

Let us learn about Cambium. After reading this article you will learn about: 1. Origin of Cambium 2. Fascicular and Inter-fascicular Cambium 3. Duration 4. Functions 5. Structure 6. Cell Division 7. Thickening in Palms.

Origin of Cambium:

The primary vascular skeleton is built up by the maturing of the cells of the procambium strands to form xylem and phloem. The plants which do not possess secondary growth, all cells of the procambium strands mature and develop into vascular tissue.

In the plant which have secondary growth later on, a part of the procambium strand remains meristematic and gives rise to the cambium proper. In roots the formation of cambium differs from that in stems because of the radial arrangement of the alternating xylem and phloem strands.

Here the cambium arises as discrete strips of tissue in the procambium strands inside the groups of primary phloem. Later on, the strips of cambium by their lateral extension are joined in the pericycle opposite the rays of primary xylem. The secondary tissue formation is most rapid beneath the groups of phloem so that the cambium, as seen in the transverse section of older roots, soon forms a circle.

Fascicular and Inter-fascicular Cambium:

In stems the first procambium that develops from promeristem is usually found in the form of isolated strands. In some plants these first-formed strands soon become, united laterally by additional similar strands formed between them and by the lateral extention of the first-formed strands.

During further development this procambial cylinder gives rise to a cylinder of primary vascular tissue (xylem and phloem) and cambium. Later on, a cylinder of secondary vascular tissue is formed that arises in strands as does the primary cylinder. In Ranunculus and some other herbaceous plants, the procambium strands, and the primary vascular tissues, do not fuse laterally but remain as discrete strands.

More often in herbaceous stems the cambium extends laterally across the intervening spaces until a complete cylinder is formed. Where such extension occurs, the cambium arises from inter-fascicular meristematic cells derived from the apical meristem.

The strips of cambium that arise within collateral bundles are known as fascicular cambium, and the cambial strips found in between the bundles are known as inter-fascicular cambium.

Duration of Cambium:

The duration of the functional life of the cambium varies greatly in different species and also in different parts of the same plant. In a perennial woody plant the cambium of the main stem lives from the time of its formation until the death of the plant.

It is only by the continued activity of the cambium in producing new xylem and phloem that such plants can maintain their existence. In leaves, inflorescenes and other deciduous parts, the functional life of the cambium is short. Here all the cambium cells mature as vascular tissue. The secondary xylem is directly found upon the secondary phloem in such bundles.

Function of Cambium:

The meristem that forms secondary tissues consists of an uniseriate sheet of initials that form new cells usually on both sides. The cambium forms xylem internally and phloem externally. The tangential division of the cambial cell forms two apparently identical daughter cells.

One of the daughter cells remains meristematic, i.e., the persistent cambial cell, the other becomes a xylem mother cell or a phloem mother cell depending upon its position internal or external to the initial. The cambium cell divides continuously in a similar way one daughter cell always remains meristematic, the cambium cell, whereas the other becomes either a xylem or a phloem mother cell.

Probably there is no definite alternation and for brief periods only one kind of tissue is formed. Adjacent cambium cells divide at nearly the same time, and the daughter cells belong to the same tissue. This way, the tangential continuity of the cambium is maintained.

Structure of Cambium:

There are two general conceptions of the cambium as an initiating layer:

1. That it consists of a uniseriate layer of permanent initials with derivatives which may divide a few times and soon become converted into permanent tissue

2. That there are several rows of initating cells which form a cambium zone, a few individual rows of which persist as cell forming layers for some time. During growing periods the cells mature continuously on both sides of the cambium it becomes quite obvious that only a single layer of cells can have permanent existence as cambium.

Other layers, if present, function only temporarily and become completely transformed into permanent cells. In a strict sense, only the initials constitute the cambium, but frequently the term is used with reference to the cambial zone, because it is difficult to distinguish the initials from their recent derivatives.

Cellular Structure of Cambium:

There are two different types of cambium cells:

1. The ray initials, which are more or less isodiametric and give rise to vascular rays and

2. The fusiform initials, the elongate tapering cells that divide to form all cells of the vertical system.

The cambial cells are highly vacuolated, usually with one large vacuole and thin peripheral cytoplasm. The nucleus is large and in the fusiform cells is much elongated. The walls of cambial cells have primary pit fields with plasmodesmata. The radial walls are thicker than tangential walls, and their primary pit fields are deeply depressed.

Cell Division in Cambium:

With the result of tangential (periclinal) divisions of cambium cells the phloem and the xylem are formed. The vascular tissues are formed in two opposite directions, the xylem cells towards the interior of the axis, the phloem cells toward its periphery. The tangential divisions of the cambium initials during the formation of vascular tissues determine the arrangement of cambial derivatives in radial rows.

Since the division is tangential, the daughter cells that persist as cambium initials increase in radial diameter only. The new cambium initials formed by transverse divisions increase greatly in length those formed by radial divisions do not increase in length.

As the xylem cylinder increases in thickness by secondary growth, the cambial cylinder also grows in circumference. The main cause of this growth is the increase in the number of cells in tangential direction, followed by a tangential expansion of these cells.

Cambium Growth about Wounds:

One of the important functions of the cambium is the formation of callus or wound tissue, and the healing of the wounds. When wounds occur on plants, a large amount of soft parenchymatous tissue is formed on or below the injured surface this tissue is known as callus. The callus develops from the cambium and by the division of parenchyma cells in the phloem and the cortex.

During the healing process of a wound the callus is formed. In this there is at first abundant proliferation of the cambium cells, with the production of massive parenchyma. The outer cells of this tissue become suberized, or periderm develops within them, with the result a bark is formed.

However, just beneath this bark the cambium remains active and forms new vascular tissue in the normal way. The new tissue formed in the normal way extends the growing layer over the wound until the two opposite sides meet. The cambium layers then unite and the wound becomes completely covered.

Cambium in Budding and Grafting:

In the practices of budding and grafting, the cambium of both stock and scion gives rise to callus which unites and develops a continuous cambium layer that gives rise to normal conducting tissue. There is an actual union of the cambium of stock and scion of two plants during the practices of budding and grafting and therefore these practices are not commonly found in monocotyledons.

Cambium in Monocotyledons:

A special type of secondary growth occurs in few monocotyledonous forms, such as Dracaena, Aloe, Yucca, Veratrum and some other genera. In these plants the stem increases in diameter forming a cylinder of new bundles embedded in a tissue.

Here a cambium layer develops from the meristematic parenchyma of the peri-cycle or the innermost cells of the cortex. In the case of roots, the cambium of this develops in the endodermis. The initials of cambium strand in tiers to form a storied cambium as found in the normal cambium of some dicotyledons.

Cambium in Thickening in Palms:

The palm stems do not increase in girth, because of any cambial activity but this thickening is the result of gradual increase in size of cells and of intercellular spaces and sometimes of the proliferation of fibre tissues. This is the type of long continuing primary growth.

The process is as follows:

Most of the monocotyledons lack secondary growth, but with the result of intense and long continuing primary growth they may produce such large bodies as those of the palms. The monocotyledons often produce a rapid thickening beneath the apical meristem by means of a peripheral primary thickening meristem as shown in figure.

The activity of the primary thickening meristem resembles with secondary growth found in certain monocotyledons such as Dracaena, Yucca, etc. The apical meristem also known as shoot apex produces only small part of the primary body, i.e., a central column of parenchyma and vascular strands.

Most of the plant body is formed by the primary thickening meristem. The primary thickening meristem is found beneath the leaf-primordia, which divides periclinally producing anticlinal rows of cells. These cells differentiate into a tissue formed of ground parenchyma traversed by procambial strands.

These procambial strands later on develop into vascular bundles. The ground parenchyma cells enlarge and divide repeatedly, causing increase in thickness. This way, both apical meristem and primary thickening meristem give rise to the main bulk of the stem tissues of monocotyledons.

The thickening takes place in monocotyledons, such as palms, due to the activities of the apical meristem and primary thickening meristem.

Salivary glands and saliva composition

The salivary glands of aphids are paired and the right and left glands have two glandular units, a large principal gland and a smaller accessory gland. The salivary ducts of both glandular units on one side join together and then their common duct joins the one coming from the contralateral side. The principal gland is innervated and contains eight secretory cells, possibly secreting different components ( Ponsen, 1972). This gland seems to play a major role in the sheath saliva production. The accessory gland does not appear to be enervated, and its cells do not show much differentiation. Transmission studies of persistent/circulative plant viruses have shown that the accessory glands transfer the virus from the haemolymph to the salivary canal in the stylets and into plants ( Gray and Gildow, 2003). From this, it has been inferred that the watery E1 saliva must come from the accessory glands since E1 salivation is responsible for inoculation of these viruses ( Prado and Tjallingii, 1994). It remains unclear whether the principal glands exclusively produce the sheath saliva and the accessory glands the watery saliva. It cannot be excluded that saliva composition comes from both glands. Although there is no experimental evidence, the innervation of the principal gland suggests that aphids may adjust at least the saliva contribution from this gland on the basis of gustatory information.

Recently, Peter Miles (1999) reviewed the current aphid saliva knowledge. The published protein components show a lot of contradictions, not only between but also within aphid species ( Miles and Harrewijn, 1991 Baumann and Baumann, 1995 Urbanska et al., 1994 Madhusudhan and Miles, 1998 Cherqui and Tjallingii, 2000 Kornemann, 2005). Parafilm® covered diets have mostly been used to collect saliva. Possibly, on the basis of the sensory aspects mentioned above, the saliva composition might vary due to the different diet compositions. Moreover, using diet collection, sheath saliva has inevitably been mixed with watery saliva while E1 and E2 salivation periods are mostly short in fluid diets. Sampling saliva from separate behavioural phases is the most difficult aspect of salivary research. It cannot be excluded that within watery saliva, the composition differs between pd-salivation, E1 salivation, and E2 salivation.

Stem Anatomy

The stem and other plant organs are primarily made from three simple cell types: parenchyma, collenchyma, and sclerenchyma cells. Parenchyma cells are the most common plant cells. They are found in the stem, the root, the inside of the leaf, and the pulp of the fruit. Parenchyma cells are responsible for metabolic functions, such as photosynthesis. They also help repair and heal wounds. In addition, some parenchyma cells store starch.

Figure (PageIndex<1>): Parenchyma cells in plants: The stem of common St John&rsquos Wort (Hypericum perforatum) is shown in cross section in this light micrograph. The central pith (greenish-blue, in the center) and peripheral cortex (narrow zone 3&ndash5 cells thick, just inside the epidermis) are composed of parenchyma cells. Vascular tissue composed of xylem (red) and phloem tissue (green, between the xylem and cortex) surrounds the pith.

Collenchyma cells are elongated cells with unevenly-thickened walls. They provide structural support, mainly to the stem and leaves. These cells are alive at maturity and are usually found below the epidermis. The &ldquostrings&rdquo of a celery stalk are an example of collenchyma cells.

Figure (PageIndex<1>): Collenchyma cells in plants: Collenchyma cell walls are uneven in thickness, as seen in this light micrograph. They provide support to plant structures.

Sclerenchyma cells also provide support to the plant, but unlike collenchyma cells, many of them are dead at maturity. There are two types of sclerenchyma cells: fibers and sclereids. Both types have secondary cell walls that are thickened with deposits of lignin, an organic compound that is a key component of wood. Fibers are long, slender cells sclereids are smaller-sized. Sclereids give pears their gritty texture. Humans use sclerenchyma fibers to make linen and rope.

Figure (PageIndex<1>): Sclerenchyma cells in plants: The central pith and outer cortex of the (a) flax stem are made up of parenchyma cells. Inside the cortex is a layer of sclerenchyma cells, which make up the fibers in flax rope and clothing. Humans have grown and harvested flax for thousands of years. In (b) this drawing, fourteenth-century women prepare linen. The (c) flax plant is grown and harvested for its fibers, which are used to weave linen, and for its seeds, which are the source of linseed oil.

As with the rest of the plant, the stem has three tissue systems: dermal, vascular, and ground tissue. Each is distinguished by characteristic cell types that perform specific tasks necessary for the plant&rsquos growth and survival.


One of the crucial functions of phloem is to transport the products of photosynthesis from sources to sinks. The morphology of the connection determines how photoassimilates produced in the mesophyll cells are loaded into the SE-CC complex for further long-distance transport. Three major strategies of phloem loading are currently recognized: active, symplastic loading known as “polymer trap” active loading through the apoplast and the less well-studied passive transport though the symplast ( Slewinski et al., 2013 ). The SE-CC complex described by Shih and Currier (1969) most probably represents “polymer trap” phloem loading. Numerous symplastic connections enable the diffusion of sucrose from the photosynthetic mesophyll cells to the companion cells there, it is converted to raffinose and stachyose, oligosaccharides that cannot diffuse back across the PD due to their large size. Synthesis of these polymers in the CCs serves two important goals. First, it maintains a low concentration of sucrose in the CC, which in turn propels further uptake of sucrose from the surrounding mesophyll cells. Second, the increased accumulation of polymers in the SE-CC complex elevates the hydrostatic pressure, which enables long-distance transport along the phloem SEs ( Turgeon et al., 1993 Zhang and Turgeon, 2009 ).

An entirely different loading strategy is used by plants of Arabidopsis, among others. In “apoplastic loading”, sucrose is symplastically transported from the photosynthetic mesophyll cells to the phloem parenchyma transfer cells, where it is actively secreted to the apoplast companion cells then actively take up sucrose from the apoplast. Active sucrose uptake has long been known to be mediated by membrane-localized H + -coupled sucrose symporters ( Haritatoset al., 2000 Wippel and Sauer, 2012 ). However, a family of sucrose efflux carriers (SWEET) has only recently been identified, clarifying the mechanism of sucrose secretion to the apoplast ( Chen et al., 2010 ). Phloem parenchyma cells of apoplastically loading plants often have pronounced cell wall ingrowths at their interface with the companion cells. This irregularity of the cell wall shape is believed to be an adaptive feature that increases the apoplastic volume and the surface area of the plasma membrane hosting sucrose efflux and influx carriers. There are very limited plasmodesmatal connections from CCs to cells other than the SE, probably to prevent the leakage of imported sucrose ( Henry and Steer, 1980 Rennie and Turgeon, 2009 ).

Passive transport is a phloem loading strategy characterized by a symplastic flux of sucrose that relies on a higher concentration of sugar in the mesophyll cells than in the CC-SE complex. So far, this mechanism has mostly been observed in woody plants. In plants with passive loading, companion cells develop numerous symmetrically branched plasmodesmata to facilitate photoassimilate transfer. To maintain a sufficient sugar concentration gradient, leaves of passively loading plants have to accumulate much higher amounts of carbohydrates in their mesophyll cells than those of plants using active loading ( Rennie and Turgeon, 2009 ).

Although plants have generally been seen to use one type of phloem loading strategy, their phloem bundles may contain SE-CC complexes of different types. This composition gives the plant the flexibility to respond to changes in photosynthetic conditions or carbohydrate concentrations in the source tissue by switching between passive and active loading mechanisms ( Slewinski et al., 2013 ).

Radioactive CO2 has been used as a tracer of phloem translocation of photoassimilates since 1952, when Vernon and Aronoff (1952) reported the translocation of C14O2 in soybean. In addition, two fluorescent dyes, 5-(6)-carboxy-fluorescein diacetate (CFDA) and 8-hydroxypyrene-1,3,6-trisulfonic acid (HPTS-acetate), are used to trace flow through the phloem ( Grignon et al., 1989 ).

When continuity of the phloem network is hindered in some manner, such as wounding, a regeneration mechanism is initiated to re-establish the network. The reconnection of vascular tissues between existing and wounded phloem was described in detail in the 1980s based on the experimental interruption of phloem continuity by incision of the root stele in pea (Pisum sativum L.). After the incision, cells in the surrounding cortical layer dedifferentiated and initiated meristematic divisions. Division of activated cortical cells formed an arc surrounding the site of incision and reconnected the vasculature on both sides of the wound. The dividing cells elongated and differentiated into SE. Alongside the SE, xylem tracheary elements developed in an inner arc closer to the wound. Cambium formed between the xylem and phloem cells, ensuring future secondary growth of both types of vascular tissues ( Schulz, 1987 ). Interestingly, while xylem tracheary elements can differentiate directly from parenchyma cells without prior divisions, formation of a new SE-CC complex requires initial divisions of dedifferentiated parenchymal cells ( Nishitani et al., 2002 ).

At the ultrastructural level, SE elongation, cell wall thickening, organelle rearrangement, and enucleation were shown to occur in a similar manner during regeneration and primary phloem development. However, it remains unclear how signaling for proper division and differentiation occurs in wounded phloem. In addition, little information is available on the molecular mechanisms by which wounded phloem reconnects to existing phloem, a question that warrants further study ( Behnke and Schulz, 1980 , 1983 Schulz, 1986a , b ).

Auxin has been suggested as a potential noncellular autonomous signal inducing vascular regeneration after wounding. Removal of young leaves and buds, a natural source of auxin in the plant, strongly reduces phloem regeneration. This regeneration deficiency can be fully reverted by an exogenous auxin application ( Lamotte and Jacobs, 1963 ). Furthermore, auxin leaking from injured tissues induces cell divisions and callus formation in the wound. In addition, local application of auxin can induce the formation of new, continuous strands of vascular cells, which then connect to existing vascular bundles. Development of the induced vasculature toward the basal part of the plant corresponds with the general basipetal direction of auxin movement ( Berleth et al., 2000 ). However, it is still not fully clear whether the later steps of phloem differentiation are also controlled by auxin or by other signaling molecules delivered from the interrupted phloem strands ( Sawchuk and Scarpella, 2013 ).

Monoclonal antibodies against phloem P-protein from plant tissue cultures. II. Taxonomic distribution of cross-reactivity

P-protein, a filamentous protein found in the sieve elements of most angiosperms, is believed to function in the sealing of phloem wound sites. We report here on the use of a highly sensitive immunomicroscopy assay to study the ability of P-protein specific monoclonal antibodies RS21, RS22, and RS23, made against the P-protein from Streptanthus tortuosus (Brassicaceae), to recognize the native P-protein in a number of different plant genera. RS21, RS22, and RS23 all recognized the P-protein in other genera within the Brassicaceae including Arabidopsis and in the closely related family, Capparaceae. RS21 and RS22 also were able to bind to the P-protein in plants more distantly related to S. tortuosus. The labeling of P-protein was also observed in the monocots Iris and Narcissus probed with RS21. No label was seen with members of the Poaceae that are reported to lack P-protein. None of the monoclonal antibodies was able to bind to the P-protein in members of the Cucurbitaceae.

23.2 Stems

In this section, you will explore the following questions:

  • What is the main function and basic structure of a plant stem?
  • What are the roles of dermal tissues, vascular tissues, and ground tissues?
  • What is the difference between primary growth and secondary growth in stems?
  • What is the origin of annual rings in stems? How are annual rings used to approximate the age of a tree?
  • What are examples of modified stems?

Connection for AP ® Courses

Much content described in this section is not within the scope of AP ® . You are not required to memorize the different types of tissues that comprise the plant stem. However, in the Transport of Water and Solutes in Plants module we will explore in detail the roles vascular tissues (xylem and phloem), epidermal guard cells, stomata, and trichomes play in transpiration, the uptake of carbon dioxide and the release of oxygen and water vapor. Trichomes—hair-like structures on the epidermal surface—also defend leaves against predation (see the Plant Sensory Systems and Reponses module).

Except for the concepts described in the AP ® Connection, information presented in this module, and the examples highlighted, does not align to the content and AP ® Learning Objectives outlined in the AP ® Curriculum Framework.

Stems are a part of the shoot system of a plant. They may range in length from a few millimeters to hundreds of meters, and also vary in diameter, depending on the plant type. Stems are usually above ground, although the stems of some plants, such as the potato, also grow underground. Stems may be herbaceous (soft) or woody in nature. Their main function is to provide support to the plant, holding leaves, flowers and buds in some cases, stems also store food for the plant. A stem may be unbranched, like that of a palm tree, or it may be highly branched, like that of a magnolia tree. The stem of the plant connects the roots to the leaves, helping to transport absorbed water and minerals to different parts of the plant. It also helps to transport the products of photosynthesis, namely sugars, from the leaves to the rest of the plant.

Plant stems, whether above or below ground, are characterized by the presence of nodes and internodes (Figure 23.4). Nodes are points of attachment for leaves, aerial roots, and flowers. The stem region between two nodes is called an internode. The stalk that extends from the stem to the base of the leaf is the petiole. An axillary bud is usually found in the axil—the area between the base of a leaf and the stem—where it can give rise to a branch or a flower. The apex (tip) of the shoot contains the apical meristem within the apical bud.

Stem Anatomy

The stem and other plant organs arise from the ground tissue, and are primarily made up of simple tissues formed from three types of cells: parenchyma, collenchyma, and sclerenchyma cells.

Parenchyma cells are the most common plant cells (Figure 23.5). They are found in the stem, the root, the inside of the leaf, and the pulp of the fruit. Parenchyma cells are responsible for metabolic functions, such as photosynthesis, and they help repair and heal wounds. Some parenchyma cells also store starch.

Collenchyma cells are elongated cells with unevenly thickened walls (Figure 23.6). They provide structural support, mainly to the stem and leaves. These cells are alive at maturity and are usually found below the epidermis. The “strings” of a celery stalk are an example of collenchyma cells.

Sclerenchyma cells also provide support to the plant, but unlike collenchyma cells, many of them are dead at maturity. There are two types of sclerenchyma cells: fibers and sclereids. Both types have secondary cell walls that are thickened with deposits of lignin, an organic compound that is a key component of wood. Fibers are long, slender cells sclereids are smaller-sized. Sclereids give pears their gritty texture. Humans use sclerenchyma fibers to make linen and rope (Figure 23.7).

Visual Connection

  1. The cortex and pith are made of parenchyma cells.
  2. The companion cells of the phloem are parenchyma cells.
  3. Fiber cells of the sclerenchyma
  4. Sieve elements and tracheids of the xylem

Like the rest of the plant, the stem has three tissue systems: dermal, vascular, and ground tissue. Each is distinguished by characteristic cell types that perform specific tasks necessary for the plant’s growth and survival.

Dermal Tissue

The dermal tissue of the stem consists primarily of epidermis, a single layer of cells covering and protecting the underlying tissue. Woody plants have a tough, waterproof outer layer of cork cells commonly known as bark, which further protects the plant from damage. Epidermal cells are the most numerous and least differentiated of the cells in the epidermis. The epidermis of a leaf also contains openings known as stomata, through which the exchange of gases takes place (Figure 23.8). Two cells, known as guard cells, surround each leaf stoma, controlling its opening and closing and thus regulating the uptake of carbon dioxide and the release of oxygen and water vapor. Trichomes are hair-like structures on the epidermal surface. They help to reduce transpiration (the loss of water by aboveground plant parts), increase solar reflectance, and store compounds that defend the leaves against predation by herbivores.

Vascular Tissue

The xylem and phloem that make up the vascular tissue of the stem are arranged in distinct strands called vascular bundles, which run up and down the length of the stem. When the stem is viewed in cross section, the vascular bundles of dicot stems are arranged in a ring. In plants with stems that live for more than one year, the individual bundles grow together and produce the characteristic growth rings. In monocot stems, the vascular bundles are randomly scattered throughout the ground tissue (Figure 23.9).

Xylem tissue has three types of cells: xylem parenchyma, tracheids, and vessel elements. The latter two types conduct water and are dead at maturity. Tracheids are xylem cells with thick secondary cell walls that are lignified. Water moves from one tracheid to another through regions on the side walls known as pits, where secondary walls are absent. Vessel elements are xylem cells with thinner walls they are shorter than tracheids. Each vessel element is connected to the next by means of a perforation plate at the end walls of the element. Water moves through the perforation plates to travel up the plant.

Phloem tissue is composed of sieve-tube cells, companion cells, phloem parenchyma, and phloem fibers. A series of sieve-tube cells (also called sieve-tube elements) are arranged end to end to make up a long sieve tube, which transports organic substances such as sugars and amino acids. The sugars flow from one sieve-tube cell to the next through perforated sieve plates, which are found at the end junctions between two cells. Although still alive at maturity, the nucleus and other cell components of the sieve-tube cells have disintegrated. Companion cells are found alongside the sieve-tube cells, providing them with metabolic support. The companion cells contain more ribosomes and mitochondria than the sieve-tube cells, which lack some cellular organelles.

Ground Tissue

Ground tissue is mostly made up of parenchyma cells, but may also contain collenchyma and sclerenchyma cells that help support the stem. The ground tissue towards the interior of the vascular tissue in a stem or root is known as pith, while the layer of tissue between the vascular tissue and the epidermis is known as the cortex.

Growth in Stems

Growth in plants occurs as the stems and roots lengthen. Some plants, especially those that are woody, also increase in thickness during their life span. The increase in length of the shoot and the root is referred to as primary growth, and is the result of cell division in the shoot apical meristem. Secondary growth is characterized by an increase in thickness or girth of the plant, and is caused by cell division in the lateral meristem. Figure 23.10 shows the areas of primary and secondary growth in a plant. Herbaceous plants mostly undergo primary growth, with hardly any secondary growth or increase in thickness. Secondary growth or “wood” is noticeable in woody plants it occurs in some dicots, but occurs very rarely in monocots.

Some plant parts, such as stems and roots, continue to grow throughout a plant’s life: a phenomenon called indeterminate growth. Other plant parts, such as leaves and flowers, exhibit determinate growth, which ceases when a plant part reaches a particular size.

Primary Growth

Most primary growth occurs at the apices, or tips, of stems and roots. Primary growth is a result of rapidly dividing cells in the apical meristems at the shoot tip and root tip. Subsequent cell elongation also contributes to primary growth. The growth of shoots and roots during primary growth enables plants to continuously seek water (roots) or sunlight (shoots).

The influence of the apical bud on overall plant growth is known as apical dominance, which diminishes the growth of axillary buds that form along the sides of branches and stems. Most coniferous trees exhibit strong apical dominance, thus producing the typical conical Christmas tree shape. If the apical bud is removed, then the axillary buds will start forming lateral branches. Gardeners make use of this fact when they prune plants by cutting off the tops of branches, thus encouraging the axillary buds to grow out, giving the plant a bushy shape.

Link to Learning

Watch this BBC Nature video showing how time-lapse photography captures plant growth at high speed.

Watch this BBC Nature video showing how time-lapse photography captures plant growth at high speed.
  1. opening of a flower
  2. tendrils looping around a support
  3. growth of an apical bud
  4. closing of leaflets on a lightly touched mimosa leaf

Secondary Growth

The increase in stem thickness that results from secondary growth is due to the activity of the lateral meristems, which are lacking in herbaceous plants. Lateral meristems include the vascular cambium and, in woody plants, the cork cambium (see Figure 23.10). The vascular cambium is located just outside the primary xylem and to the interior of the primary phloem. The cells of the vascular cambium divide and form secondary xylem (tracheids and vessel elements) to the inside, and secondary phloem (sieve elements and companion cells) to the outside. The thickening of the stem that occurs in secondary growth is due to the formation of secondary phloem and secondary xylem by the vascular cambium, plus the action of cork cambium, which forms the tough outermost layer of the stem. The cells of the secondary xylem contain lignin, which provides hardiness and strength.

In woody plants, cork cambium is the outermost lateral meristem. It produces cork cells (bark) containing a waxy substance known as suberin that can repel water. The bark protects the plant against physical damage and helps reduce water loss. The cork cambium also produces a layer of cells known as phelloderm, which grows inward from the cambium. The cork cambium, cork cells, and phelloderm are collectively termed the periderm. The periderm substitutes for the epidermis in mature plants. In some plants, the periderm has many openings, known as lenticels, which allow the interior cells to exchange gases with the outside atmosphere (Figure 23.11). This supplies oxygen to the living and metabolically active cells of the cortex, xylem and phloem.

Annual Rings

The activity of the vascular cambium gives rise to annual growth rings. During the spring growing season, cells of the secondary xylem have a large internal diameter and their primary cell walls are not extensively thickened. This is known as early wood, or spring wood. During the fall season, the secondary xylem develops thickened cell walls, forming late wood, or autumn wood, which is denser than early wood. This alternation of early and late wood is due largely to a seasonal decrease in the number of vessel elements and a seasonal increase in the number of tracheids. It results in the formation of an annual ring, which can be seen as a circular ring in the cross section of the stem (Figure 23.12). An examination of the number of annual rings and their nature (such as their size and cell wall thickness) can reveal the age of the tree and the prevailing climatic conditions during each season.

Stem Modifications

Some plant species have modified stems that are especially suited to a particular habitat and environment (Figure 23.13). A rhizome is a modified stem that grows horizontally underground and has nodes and internodes. Vertical shoots may arise from the buds on the rhizome of some plants, such as ginger and ferns. Corms are similar to rhizomes, except they are more rounded and fleshy (such as in gladiolus). Corms contain stored food that enables some plants to survive the winter. Stolons are stems that run almost parallel to the ground, or just below the surface, and can give rise to new plants at the nodes. Runners are a type of stolon that runs above the ground and produces new clone plants at nodes at varying intervals: strawberries are an example. Tubers are modified stems that may store starch, as seen in the potato (Solanum sp.). Tubers arise as swollen ends of stolons, and contain many adventitious or unusual buds (familiar to us as the “eyes” on potatoes). A bulb, which functions as an underground storage unit, is a modification of a stem that has the appearance of enlarged fleshy leaves emerging from the stem or surrounding the base of the stem, as seen in the iris.

Link to Learning

Watch botanist Wendy Hodgson, of Desert Botanical Garden in Phoenix, Arizona, explain how agave plants were cultivated for food hundreds of years ago in the Arizona desert in this video: Finding the Roots of an Ancient Crop.

  1. sweetener for drinks and cooking
  2. proteins to supplement the daily diet
  3. lipids for cooking and baking
  4. starch for thickening desserts and stews

Some aerial modifications of stems are tendrils and thorns (Figure 23.14). Tendrils are slender, twining strands that enable a plant (like a vine or pumpkin) to seek support by climbing on other surfaces. Thorns are modified branches appearing as sharp outgrowths that protect the plant common examples include roses, Osage orange and devil’s walking stick.

Stem regions at which leaves are attached are called ________.

Which of the following cell types forms most of the inside of a plant?

Tracheids, vessel elements, sieve-tube cells, and companion cells are components of ________.

The primary growth of a plant is due to the action of the ________.

Which of the following is an example of secondary growth?

Secondary growth in stems is usually seen in ________.

Describe the roles played by stomata and guard cells. What would happen to a plant if these cells did not function correctly?

Stomata allow gases to enter and exit the plant. Guard cells regulate the opening and closing of stomata. If these cells did not function correctly, a plant could not get the carbon dioxide needed for photosynthesis, nor could it release the oxygen produced by photosynthesis.

Compare the structure and function of xylem to that of phloem.

Xylem is made up tracheids and vessel elements, which are cells that transport water and dissolved minerals and that are dead at maturity. Phloem is made up of sieve-tube cells and companion cells, which transport carbohydrates and are alive at maturity.

Explain the role of the cork cambium in woody plants.

In woody plants, the cork cambium is the outermost lateral meristem it produces new cells towards the interior, which enables the plant to increase in girth. The cork cambium also produces cork cells towards the exterior, which protect the plant from physical damage while reducing water loss.

What is the function of lenticels?

In woody stems, lenticels allow internal cells to exchange gases with the outside atmosphere.

Besides the age of a tree, what additional information can annual rings reveal?

Annual rings can also indicate the climate conditions that prevailed during each growing season.

Give two examples of modified stems and explain how each example benefits the plant.

Answers will vary. Rhizomes, stolons, and runners can give rise to new plants. Corms, tubers, and bulbs can also produce new plants and can store food. Tendrils help a plant to climb, while thorns discourage herbivores.

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    Supplemental Information

    Examples of non-dispersive P-protein bodies (NPBs) that do not respond to wounding of sieve elements by puncturing with micro-pipettes.

    The CLSM micrographs were taken after a sieve element had been severed with a micro-pipette arrowheads point to unresponsive NPBs. In all cases, the NPBs are located close to a sieve plate (compare Fig. 5). (A) Theobroma cacao and (B) Pombalia communis stained with aniline blue and synapto red. (C) Viola tricolor stained with CDMFDA. The micro-pipette is visible in this image (white arrows). Scale bars: 10 μm.

    Comparison of the hypothetical product of gene Potri.017G071000.1 (from Populus trichocarpa v3.0,, PtSEOR1, and AtSEOR1.

    Positions in which Potri.017G071000.1 and PtSEOR1 differ show in red, residues that are identical in the two are shown in blue. Positions in which AtSEOR1 matches the consensus of the P. trichocarpa sequences also appear in blue. The alignment was produced with CLC Sequence Viewer v. 7.8.1.

    TEM micrographs of a sieve element in an Arabidopsis leaf with bundles of SEOR protein filaments in longitudinal and perpendicular section.

    (A) cross-section of a sieve element with two slime masses consisting of filaments of SEOR protein (arrows). (B) zoom into one of the protein masses, showing longitudinal or oblique sections of the winding SEOR filaments as well as sections perpendicular to the filament axes. (C) zoom into that part of the protein mass where filaments are sectioned more or less perpendicularly. The distribution of the filament cross-sections does not follow a clear geometric pattern, indicating that the packing density does not approach its theoretical maximum. Nonetheless an early stage in the development of a hexagonal arrangement is suggested. For methods, see Froelich et al. (2011), Plant Cell 23, 4428–4445.

    Comparison of the putative promoter sequences of the P. trichocarpa genes Potri.001G430200.1 and Potri.017G071000.1, the PtSEOR1 gene.

    Identical bases appear in blue, different bases and gaps are shown in red. The 100-bp sequence that confers phloem-specificity to the Potri.001G340200.1 promoter (Nguyen et al., 2017) and the corres-ponding sequence in Potri.017G071000.1, the PtSEOR1 gene, are shown on yellow background. Of the 100 base pairs, 71 are conserved. Note the conserved TATA-box motif at position −228. The alignment was produced with CLC Sequence Viewer v. 7.8.1.

    Watch the video: IB - Transport in Phloem (January 2022).