Water and Wood part 1

How remarkable it is that the single cell that begins the life of a plant can develop in such an immense variety of ways. Looking at k a newly fertilized cell within an ovary, it is not possible to tell whether it will develop into a tiny alpine plant only a few centimetres high or a giant Californian Redwood (Sequoia sempervirens) over 100 metres tall. The beginnings are the same, but as that one cell grows and divides so the characteristics of the new plant emerge. If it is to be an annual and complete its life-cycle within a year, then there is no need for elaboration of stem cells to give strength and durability. In the case of a tree, however, the stem will develop into a trunk which must be capable of standing for several hundred years and supporting a heavy head of foliage. From the trunk will develop branches and these too will need rigidity and strength, while the central conducting tissue will have to pass water and minerals from the roots up as much as 100 metres of trunk and branches for the whole of the tree’s life.

If a transverse section is taken through the stem of any non- woody plant (that is by cutting it straight across) it is possible to see the conducting tissues. Unlike those of the root, however, they lie in small groups or strands known as vascular bundles, positioned just within the surface of the stem. In some species they lie close together to form an almost continuous ring around the central tissue, in others they are clearly separated by a layer of parenchyma. If the section is cut where a leaf joins the stalk, it will be seen that one or more strands of vascular tissue run from the stem into the leaf, these are known as leaf traces.

Each vascular bundle is made up of xylem or ligneous tissue to the inside and phloem on the outside, with a layer of cambium between. Cambium is capable of dividing and forming new cells of both sorts. Both parts of the vascular bundle contain conducting elements, that in the xylem being made up of two types of specialized cells known as vessels and tracheids. Vessels are rows of cells which become joined end to end during their development, the cross walls disappearing to create the pipe-like vessel which forms an excellent water-conducting channel. The walls of the vessels like those of the tracheids are thickened for strength, often with the thickening in a spiral. Tracheids are similar to vessels but are made up of one cell only, the cross walls making them somewhat less efficient. Once these conducting cells have become fully developed they lose their nuclei and normal cell content and are then dead, incapable of further growth or change.

The phloem is made up of sieve tubes. These too are elongated cells joined end to end, but their cross walls are perforated to allow easy passage of water from one to another. Unlike the vessels and tracheids, the outer walls of sieve tubes do not become thickened and they remain alive, the cells retaining their nuclei and other contents. The vessels, tracheids and sieve tubes are the most important parts of the conducting tissue and each bundle is contained within a protective sheath of thickened sclerenchyma. The central part of the stem, sometimes called the pith, is composed of undifferentiated parenchymatous cells, as is the tissue just beneath the epidermis, the latter containing some chlorophyll. The epidermis itself functions in much the same way as that of leaves, protecting the working cells within, and having some stomata and often hairs as well.

In the straight sections of the stem, known as internodes, the vascular bundles form continuous strands which run at an equal distance from the epidermis. The points where a leaf forms is called a node, and here the leaf traces branch off to supply the leaf with minerals and water and to obtain from it the products of photosynthesis. The way in which the leaves are arranged is very important in differentiating between genera and species.

Some develop in pairs on opposite sides of the stem, as in European Ash (Fraxinus excelsior), others alternately on either side as in elms (Ulmus), and most in spiral fashion. In this latter form of growth, typical of apples (Malus), it looks superficially as if the leaves grow alternately, but closer inspection will show that the spacing takes each leaf anything from one to two thirds of the way round the stem relative to the previous one. In a few species a number of leaves all arise from the same point forming a whorl of leaves, as in Goosegrass or Cleavers (Galium aparine).

In the monocotyledons such as grasses (Gramineue) and lilies (Liii-aceae) the arrangement of vascular bundles within the stem is quite different. This is partly because their leaves contain a large number of parallel veins, each of which is supplied by a separate leaf trace. The stem therefore contains a far greater number of bundles and these are usually found through a large part of the stem tissue. There are a few exceptions, for example Black Bryony (Tamus communis), which has an arrangement much more like dicotyledons.

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