# 4: Morphology and tissue systems- the integrated plant body

 

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collenchyma extraxylary fibers fibers mechanical support systems transfer cells Transport Systems
Fig. 1 Fig. 2 Fig. 3

General Background

Since each plant organ  will be discussed in detail elsewhere, this section is intended only to remind you of the basic arrangements of tissue systems. It is not intended to be comprehensive, and by its very nature it oversimplifies the complex and wide range of form and organization existing in the higher plants. Some terms are used without explanation. The glossary, forming an essential part of the book, should be consulted if the meaning of a term is not clear.

There are several problems which terrestrial plants have to face. These are:

1. Mechanical i.e. supporting itself in one way or another so that it can expose a suitable surface area with cells containing chloroplasts to the sunlight to intercept and fix solar energy.

2. The movement of water and minerals from the soil through the roots to regions where they can be combined with other materials to build the plant body, and the movement of synthesized food material from the site of synthesis to places of growth or storage and from the stores to growing cells.

3. Reproduction, placement of reproductive organs where the pollen or gamete receptor mechanism can operate successfully, and after fertilization and spore/seed production, ensure dispersal of the propagules.

4. Secondary growth in thickness the first two problems outlined above are dealt with by well organized, (if complex) support and transport systems in the higher plants, and will be dealt with in this Factfile. The third, reproduction, is outside the scope of this Factfile and the fourth, secondary growth, is discussed elsewhere.

Mechanical support systems

EDITING TO HERE (a) The inflated or turgid, thin-walled cells; these are present in growing points, and the cortex and parenchymatous pith of many plants. They constitute the bulk of many succulent plants, for example, Aloe, Gasleria leaves, Salicornia from salt marshes and Lithops from desert regions.

The cell wall acts as a slightly elastic container; internal liquid pressure inflates the cell so that it becomes supporting, like the air in an inflated car tyre. Its support properties depend on water pressure, so a water shortage can lead to a loss of support and wilting. Some fairly large organs can be supported by this system, but they usually rely on the additional help of devices that reduce water loss, such as a thick cuticle, and perhaps also thick outer walls to the epidermal cells, and specially modified stomata. A strong epidermis is particularly important, since it acts as the outermost boundary between the plant cells and the air. A split in the skin of a tomato, for example, rapidly leads to deformation of the fruit, or a cut in the succulent leaf of a Crassula or Senceio rapidly opens up. Not many plants rely on the turgid cell and strong epidermis principle alone

(b) Both monocotyledons and dicotyledons have specially developed, elongated, thick-walled fibres, in suitable places, which assist in mechanical support. Alternatively, they have especially thick-walled parenchyma ( also called prosenchyma) or, in those primary parts of the stem where growth in length is continuing, collenchyma cells. Although there are only a few common ways in which specialized mechanical supporting cells are arranged in the stem, leaf or root, it is the variations on these themes which are of particular interest to those who have to identify small fragments of plants, or make comparative, taxonomic studies. Obviously, to be effective the mechanical system must be economical in materials, and the cells must not be arranged in such a way as to hinder or impede the essential physiological functions of the organs. Click here to return to the INDEX

The mechanical systems develop with the early growth of the seedling. Whilst turgid cells are the only means of support at first, collenchyma may rapidly become established, particularly in dicotyledonous plants. This tissue is concentrated in the outer part of the cortex, and is frequently associated with the midrib of the leaf blade, and the petiole.

Collenchyma is essentially the strengthening tissue of primary organs, or those undergoing their phase of growth in length. The cells making up this tissue have thickened cellulosic walls, are rich in pectin and are often found with chloroplasts in their living protoplasts. Click here to return to the INDEX

Sometimes the only other mechanical support is provided by the wood (xylem) tracheids of the vascular system, as in most gymnosperms, or by the tracheids, vessels and xylem fibers of the angiosperms. However, far more common are the fibres outside the xylem (extraxylary fibers) which are arranged in strands or a complete cylinder, such as in Pelargonium which can give considerable strength to herbaceous, and particularly herbaceous monocotyledon stems and leaves. The much elongated fibers, with their cellulose primary and lignified secondary walls are not so flexible and do not stretch as readily as does collenchyma; consequently they are often found most fully developed in those parts of organs that have ceased growth in length. Click here to return to the INDEX

Figure.1 shows some fiber arrangements in monocotyledon stems and leaves. In the leaf, fibers commonly strengthen the margins (e.g. Agave) and are found as girders or caps associated with the vascular bundles. In the stem, strands next to the epidermis can act rather like the iron or steel rods in reinforced concrete. Together with a ribbed outline that they often confer on the stem section, they produce a rigid yet flexible system with economy of use of strengthening material (Fig.1. ). 

Fig.1. Some mechanical systems in monocotyledons. A fleshy leaf of Gasteria; note lack of sclerenchyma in the section B. C a mesic monocotyledon, C-D shows one type of sclerenchyma arrangement in leaf TS; E-F show three of the main types of sclerenchyma arrangements in the stem TS; G-H shows a typical root section in which most strength is concentrated in the centre. en endodermis; gt, ground tissue, which may be lignified. Adapted from: Cutler,  Botha and Stevenson, Applied plant anatomy.

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Tubes are known to resist bending more effectively than solid rods of similar diameter; they also use much less material than the solid rod. It is not surprising then, that tubes or cylinders of fibers commonly occur in plant stems. They may be next to the surface, further into the cortex, or may occur as a few layers of cells uniting an outer ring of vascular bundles (Fig.1).

Mention must be made here that in some monocotyledon stems, individual vascular bundles, scattered throughout the stem, can each be enclosed in a strong cylinder of fibers, which form a bundle sheath. Each bundle plus its sheath then acts as a reinforcing rod set in a matrix of parenchymatous cells.

Fibres or sclereids in dicotyledon leaves are also often related to the arrangement of the veins in the lamina and to the petiole vascular traces. These are shown in Fig. 2. The concentration of strength in an approximately centrally placed cylinder or strand in the petiole permits considerable torsion or twisting to take place as the leaf blade is moved by the wind, without damage occurring to the delicate conducting tissues. Primary dicotyledonous stems may have fibres in the cortex and phloem. The subterranean roots of both monocotyledons and dicotyledons have to resist different forces and stresses from those imposed on the aerial stems. These are tensions or pulling forces, as opposed to bending forces. The concentration of strengthening cells near the root centre gives it rope like properties ( Fig 2.)

 

Fig 2. Some mechanical systems in dicotyledons. A schematic plant with position of the sections indicated. Liquid pressure occurs in turgid cells throughout the plant. Collenchyma is often conspicuous in actively extending regions and petioles. Sclerenchyma fibers are most abundant in parts which have ceased main extension growth. Xylem elements with thick walls have some mechanical function in young plants and give a great deal of support in most secondarily thickened plants.  Adapted from: Cutler, Botha and Stevenson, - Applied plant anatomy. Click here to return to the INDEX
 

The transport systems

It is not possible to present a simple, comprehensive model to demonstrate the wide range of arrangements of vascular systems that occur in either dicotyledon or monocotyledons. In dicotyledons that are composed of wholly primary tissues, tend to be a little more stereotyped than monocotyledons, but even then there is a very wide range of arrangements.

The essential elements of both systems are the xylem, concerned with transport of water and dissolved salts, and the phloem, which translocates synthesized but soluble materials around the plant, to places of active growth or regions of storage.

In the apex of the shoot and root, where vascular tissue is not yet developed, soluble materials and water move from cell to cell in these relatively unspecialized zones through plasmodesmata. Not far back from these growing points, however, more formal conducting systems are needed to cope with the flow of assimilate and water. Procambial strands, the precursors of the vascular bundles, are first seen and then, further from the tips, differentiation of protophloem alone followed by protoxylem and then by the metaphloem and metaxylem that together constitute the primary vascular tissues, occur together. In most dicotyledons, the newly formed strands join the previously formed vascular bundles through a leaf or branch gap.

In most dicotyledons the leaf lamina has a midrib to which are connected the lateral veins. The latter form a network composed of major and minor systems. The midrib is directly connected to the petiole trace. This enters the stem and joins into the main stem system through a leaf trace gap as described above. In the primary stem, all vascular bundles are separate from one another (indeed, they remain separate in many climbers, e.g. Cucurbita, Ecballium), but in most dicotyledons, the bundles become joined into a cylinder by growth of secondary xylem and phloem from the fascicular and interfascicular cambia.

A complex rearrangement of tissues takes place in the primary plant where the systems of the stem and root meet. In the stem vascular bundles, the phloem is normally to the outer side of the xylem in the majority of plants. In the root, the xylem is central, and may have several lobes or poles, with the phloem situated between these. The transition region between stem and root is called the hypocotyl. After secondary growth has taken place, this complex zone becomes surrounded by secondary xylem and phloem, and the shoot and root anatomy become more similar.

Transfer cells are specialized parenchymatous cells found in various parts of the plant, but in particular, in regions where there is a physiological demand for transport, but where more normal phloem or xylem cells are not in evidence. A good example is the junction between cotyledons and the shoot axis in seedlings. Transfer cells may also be present near the extremities of veins, or near to adventitious buds, for example. Thin sections of the walls of transfer cells show them to have numerous small projections directed towards the cell lumen. These greatly increase the plasmalemma-cell wall interface a site of metabolic activity concerned with the rapid, energy-mediated movement of materials between adjacent cells. The projections are so fine that conventional sections with a rotary microtome are too thick for them to be seen.

Monocotyledons are quite different from dicotyledons in terms of their vasculature. Leaf and stem are commonly much less readily separable as distinct organs, thus jointly constitute the shoot. There is no secondary growth by a true cambium, so a cylinder of vascular tissue does not form. When secondary growth occurs, as in Agave and Cordyline, it is by means of specialized tissue, situated near to the stem surface, which forms complete, individual vascular strands and additional ground tissue. Click here to return to the INDEX

Vascular bundles are usually arranged in the stem with the xylem pole facing towards the stem centre (but this is not invariably so). The arrangement of leaf vascular bundles is very variable. Grasses and Juncus species, for example, often have one row of bundles (Fig. 3.).

Fig. 3. Juncus bufonius Leaf, T.S., x 48, showing 1 row of vascular bundles, with the xylem poles directed towards the adaxial surface. Note the the marginal sclerenchyma strands and the difference in size between adaxial and abaxial epidermal cells. Each small vascular bundle has a parenchyma sheath; in larger bundles sclerenchyma caps interrupt the parenchyma sheath. From: Cutler,  Botha and Stevenson, Applied plant anatomy. Click here to return to the INDEX

 
Since there is no vascular cylinder in the stems of monocots, where leaf traces (bundles) enter the stem they do not form gaps. They may join at nodes, where all the bundles at that particular level of the stem form a type of plexus, as in Aloes. Sometimes, in stems with nodes, the leaf traces may continue downwards from their points of entry into the stem for a complete internode before joining the nodal plexus below, (e.g. Restio, Leptocarpus, Restionaceae). In other plants without nodes, (e.g. Palms) the leaf traces follow a simple path curving inwards towards the stem centre, and then gradually ‘move’ towards the outer region of the stem lower down. These leaf traces join onto the main bundles by small, inconspicuous bridging bundles. This system is beautiful in its simplicity, but very difficult to analyze because there are so many (several hundred) vascular bundles even in the narrow portion of a stem of a small palm like Rhapis. As one follows the course of bundles in a palm, they are seen to spiral down the stem.

The main root does not often develop in monocotyledons. Its function is usually taken over by numerous adventitious roots that arise at an early stage, and join the stem vascular system in what frequently appears as a jumble of vascular tissue with very short elements both in the phloem and xylem.

 

Further reading

Recommended Text:

Cutler, D F, Applied plant anatomy. London : Longman, 1978. - ISBN 0-582-44128-5

General advanced texts

Cutter, E. G.,1969. Plant Anatomy: Experiment and Interpretation. Part I Cells and Tissues. 197n Part 2, Organs. Edward Arnold, London.

Esau, K.,1965. Plant Anatomy (2nd edn), John Wiley & Sons Inc., New York.

Fahn, A., 1974. Plant Anatomy (2nd edn), Pergamon Press, London.

Foster, A. S. & Gifford, E. M., 1974. Comparative Morphology of Vascular Plants (2nd edn), W. H. Freeman & Co., San Francisco.

Books containing informative scanning electronmicrographs

Meylan, B. A. & Butterfield, B. G., 1972. Three Dimensional Structure of Wood, Chapman & Hall, London.

Troughton, J.H. & Donaldson, L.A., 1972. Probing Plant Structure, Chapman & Hall, London.

Troughton, J. H. & Sampson, F. B., 1973. Plants, A Scanning Electron Microscope Survey, John Wiley & Sons, Australasia Pty. Ltd., Sydney.