Plant stem structure varies greatly, but the following extract from Cutler Botha and Stevenson provides good working background1. An epidermis delimits primary stems, which is often very similar to that of the leaf of the same species. This is followed internally by cortical tissues, the outer layers of which, together with the epidermal cells, may contain chloroplasts. Some cells acting as a physiological boundary between the cortex and stele are often present, forming a cylinder. They may be morphologically distinct as an endodermis, but sometimes they cannot be discerned as a separate layer.
Strengthening tissues can be present in the cortex or around the periphery of the stele (usually associated with the phloem), or in both positions. These tissues are usually in the form of axially arranged, rod-like groups of cells, with gaps between them. Only in stems with very limited growth in thickness do they form a complete cylinder, and this only when primary and secondary growth have ceased. A good example of the latter is to be found in Pelargonium species, where the inner limit of the cortex is clearly demarcated by a ring of sclerenchymatous perivascular fibres.
The vascular bundles can take up a variety of arrangements. In dicotyledons they usually occupy one ring, just to the inner side of the cortex. In monocotyledons they may form one ring, or may appear to be scattered in several to many rings, or lie without apparent order in the central ground tissue. The possession of several rings of vascular bundles is not the prerogative of monocotyledons. Several families of dicotyledons have this type of arrangement, notably those with climbing members, and also in the Piperaceae. When vascular bundles are not scattered, the centre of the stem is usually parenchymatous, which may be lignified in mature stems.
The cross-section of a primary stem may have a more or less angular to circular outline. However, it can take on one of a wide range of forms, some of which assist in the identification of a family, as in Labiateae, where the section is square or may help to distinguish genera for example many Carex species have stems with a triangular section. Often the outline is modified near to nodes or in regions of leaf insertion. Sometimes a wing or ridge of tissue in line with a petiole may continue down the internode as in, for example, Lathyrus. In general, the outline of the section taken in the middle of an internode would be described for comparative purposes.
Many stems have all or most of the following tissues, working from the outside inwards: epidermis, hypodermis, cortex (with both collenchyma and chlorenchyma, or either), an endodermoid layer (or a well-defined starch sheath), vascular bundles in one or more rings, or apparently scattered, and a central ground tissue or pith. Sometimes a pericycle can be distinguished, but this is normally regarded as part of the phloem. A true endodermis with casparian strips is rarely present.
Fig. 1 Vascular bundle types from stems. A, Cucurbita pepo, diagram of bicollateral bundle, x 15. B, Piper nigrum, diagram of collateral bundle; cambium remains fascicular, x 15. C, Chondropetalum marlothii, detailed drawing of collateral bundle, lacking cambium, x 110. D, Juncus acutus detailed drawing of amphivasal bundle, x 220. c, cambium; scl, sclerenchyma.
Primary roots have not been the subject of as many, or such full studies as has been the case of stems or leaves. They do, however, show a wide range of variation which is influenced both by environment, in terms of ecological adaptation, as well as by the genotype. Compared with stems and leaves, root fragments can be difficult to identify in the primary state. This is not entirely because they are relatively undescribed or poorly represented in reference microscope slide collections, but partly since there is, overall, less variation.
In all except aerial roots and the non-anchored roots of aquatic plants, root hairs are usually present a short distance from the growing apex. These develop from the rhizodermis or root epidermis. Often the hairs arise centrally from the basal part of the cell; occasionally they arise from near one end. Again, whilst many root hair bases are level with other cells in the rhizodermis, in other plants they may be bulbous and protrude; they can be sunken into the outer cortical tissues (e.g. Stratiotes).
The cortex is sufficiently variable to be used to assist in identification. Unfortunately from that point of view, the various types of cell arrangement seem to have more ecological than systematic significance.
The vascular system
Figure 2 illustrates aspects of the structure of the cortex in roots. Note that young roots have a broad outer region, which is mostly parenchymatous, extending from just beneath the epidermis (or the hypodermis if this layer is present) to the endodermis. The endodermis is the innermost layer of the cortex, and acts as the physiological boundary between the cortex and the enclosed vascular tissue within the stele.
Fig. 2 Roots in T.S. A-C Juncus acutiflorus, A, diagram; B, lacunate cortex, x 54; C, root hair, x 218. D-F Cattleya granulosa, D, diagram; E, velamen, x 68; F, ‘solid’ cortex x 68.
The cortex on its inner side abuts onto the endodermis. This characteristic, physiologically active tissue is frequently one layer thick but in some plants it can be two or more layered. Although the endodermis can be composed of cells with evenly thickened walls, in the majority of plants the inner and anticlinal walls are more heavily thickened with lignins and suberins than the outer periclinal walls. Consequently, in T.S. they are readily distinguished from adjacent cell layers, the so-called U-shaped thickenings making them conspicuous.
Fig. 3 Root endodermis Iris sp. A, low power, sector of root T.S., x 20 B, detail from A, x 290. c, cortex; en, endodermis; ep, epidermis; oc, outer cortex; p, passage cell; ph, phloem; px, protoxylem.
Fig. 4 Some root vascular systems, all but C, x 300, C, x 35. A, Ranunculus acris tetrarch root. B, Echinodorus cordifolius diarch root. C, D, Juncus acutiflorus polyarch rot. ca, casparian strip; en, endodermis; mx, metaxylem; p, passage cell; peri, pericycle; ph, phloem
Xylem and Phloem Structure and function
The structure of the xylem and phloem in higher plants has been reviewed in a number of excellent texts and we do not propose to undertake more than to highlight some aspects that we consider relevant to this text here. We therefore advise readers to refer to any of the many excellent texts that exist in the literature.
Clearly the evolution of the conducting system, together with the development of the lignin synthesis pathway, must be amongst the important factors which contributed to the evolution of vascular plants.
Physiologically, the elements within the xylem and phloem can act independently of one another, yet the phloem relies on water provided by the xylem in order to provide the driving force required for long-distance translocation of assimilated material. Structurally, many of the elements within the xylem (with the exception of parenchymatous elements) are dead at maturity and have a highly modified wall structure. In contrast, the phloem (with the possible exception of sclerenchymatous elements) consists of cells which contain protoplasts at maturity. Sieve elements, including the more primitive sieve cells which occur in gymnosperms are unique, in that they either lack nuclei or contain only remnants of nuclei, which are of unknown functional or regulatory capacity.
Transport through the xylem is driven in part by root pressure and by the evapotranspirative processes which take place mostly through stomata, lenticels and cracks in cuticular layers. Transport in the phloem on the other hand, relies on a build-up of solutes (loaded into the sieve tubes at the sources) and the subsequent attraction of solvent to this area. Increasing pressure and the resultant enhancing of flow within the sieve tubes but away from the source, to some local or distant regions of the plant, (termed sinks) where the solutes are unloaded and utilized, is termed translocation.
In the primary plant body, xylem and phloem generally occur either in vascular bundles, (in leaves and stems) or in strands, with xylem and phloem on alternating radii in roots. In plants that have undergone secondary thickening, the xylem and phloem in roots and stems become spatially separated, but interconnected through a series of secondary rays, which are usually parenchymatic. Whilst the interrelationships are easier to follow in primary vascular tissues, clearly the development of radially-arranged ray tissues are of paramount importance in the regulation of solute and solvent transport. One could ask why doe these disparate systems occur in close proximity. Clearly, the xylem does not require any direct inputs from the phloem, but, does the phloem require or obtain any input from the xylem? Examination of the leaf blade bundles in gymnosperms and angiosperms, demonstrates close spatial relationships between the tissues. Functional sieve tubes may occur adjacent to tracheary elements, in many monocotyledonous plants, particularly amongst the grasses and sedges. These sieve tubes are the last to differentiate and mature, and curiously, they have thick walls, which in some instances (barley and wheat) have been reported to undergo lignification. Even more curious, is the lack of the identifiable companion cell-sieve tube complex, found in the early metaphloem in these plants, and commonly in all other vascular bundles in angiosperms.
Perhaps the answers to the spatial proximity of the xylem to the phloem, lies in the physiological requirements for successful phloem loading at the source, the maintenance of long-distance transport and the unloading process in local and distant sinks elsewhere in the plant. Fig 5.32 illustrates the difference in size between loading, transport and unloading phloem sieve tubes and companion cells in Nymphoides
. In the root, the mature metaphloem is about 5-10 µm in cross section, and the companion cells are 15 to 40 µm in cross section; the companion cell’s size increase reflecting its role in the phloem unloading process.
Fig. 5 A, fibre, B, tracheid and , C, vessel, contrasted; intermediate cell types exist between each.
Fig. 6 A range of vessel element perforation plates and wall pitting, all x 218. A, Camellia sinensis scalariform. B, Liriodendron tulipfera, scalariform; pits opposite. Sambucus nigra, simple plate, pits alternate. D, Euphorbia splendens, simple plate, pits alternate. E, Scirpodendron chaeri, scalariform plate, pits opposite (from primary xylem).