Water and Wood part 2

Generally, however, the vascular bundles in a straight piece of grass stem — Maize (Zea mays) being a good example — do not run parallel to the sides but weave from the inner part of the stem to the outer, returning inwards after the leaf traces have branched off. So the vascular tissue forms a series of spirals through the stem. As well as having a different arrangement in the stem, these vascular bundles are different in their individual make-up, there being no layer of cambium between the xylem and the phloem. This means that they cannot develop a woody, strengthening tissue as can dicotyledonous plants. There are exceptions, however, as in the palms and allied woody-stemmed monocotyledons.

The performance of the conducting tissue is controlled by both the water pressure in the roots and transpiration in the leaves. The water stream set up between these two carries minerals in solution from the roots to all parts including leaves and flowers. The speed at which this solution of minerals and water moves through the plant varies greatly from one type of plant to another, being generally faster in herbaceous plants than in woody ones. In certain climbers the speed of flow can be as great as 100 metres an hour but the average is between 5 and 20 metres an hour. In broad-leaved trees the rate is usually between 1 and 4 metres an hour while in mostconiferous trees it is less, usually from 20 to 40 centimetres an hour.

This low figure is because transpiration from the needle and scale leaves is so much smaller than those of a broad-leaved tree. These are the figures obtained at the height of the growing season when the plants are in full leaf. In winter the whole system works at a very much slower rate. Experiments carried out in Germany on water content and transpiration showed that it is possible for a plant to lose from its leaves in a day twelve times as much water as it can hold at one time. This gives some indication of the volume which must be entering the roots and passing through the conducting tissue each day to replace the water lost. When plants, especially climbers, are damaged or pruned early in the growing season, the strength of the flow can be seen in the amount of sap which will ooze or even drip from the wounds. A late-pruned vine can lose a great deal of water in this way.

Although the basic water flow is from roots to leaves, there are also substances which are passed between other parts of the plant body, sometimes moving against the general flow. When the young leaves first emerge in spring they are unable to manufacture their own food. Until they can do so, all their supplies have to be sent to them from the leaves which are already functioning. When the leaves mature, they will be able to return surplus products for use in other parts of the plant. When flower buds and fruit develop, organic foods are sent to them from all parts of the plant. All these varied movements are handled by the conducting tissues. In autumn, trees store starch in the cells within their woody layers, other plants within fleshy roots or bulbs. In spring this starch is broken down and used, surplus sugars sometimes being produced. In the case of the Sugar Maple (Acer saccharum) this surplus sugar is extracted by man and concentrated by boiling, to become maple syrup. Once again the conducting tissue is the transport system.

For the majority of soft-stemmed annual and perennial plants the amount of strengthening provided by the sclerenchyma in and around the vascular bundles is sufficient to hold the stems erect. This is of course vital in keeping the leaves near sunlight and the flowers and fruit in the best position for pollination and dispersal. In severe weather large plants can be blown over and damaged but this is a rare occurrence in the wild. It is, however, seen frequently in gardens where plants have been bred to produce trusses of flowers too large for the stems to be able to carry without support. In wild plants, such a heaviness of head is compensated by extra strengthening in the stems and this is seen in its most extreme form in the woody trunks of large trees. This extra thickening increases as the weight of branches and leaves becomes ever greater and it has been found that the average girth of a growing tree is in direct proportion to the number of leaves in its canopy. Firs (Abies) and spruces (Picea) with stems 40 cm in diameter have about 15 million needles. When the stem diameter has increased from 60 —70 cm, the number of needles has similarly grown to 30 — 40 million.

The cells responsible for providing increased rigidity and strength in the stem are the cambium cells which lie in the centre of each vascular bundle between xylem and phloem. The strengthened tracheids and vessels of the xylem are sufficiently firm for short-lived plants, but in the case of shrubs and trees the cambium cells continue to divide. New cells formed on the inner side become more xylem and on the outer, phloem. Much more xylem than phloem is produced, adding to the plant’s strength, and the cambium cells increase in number to compensate for the ever-increasing volume of woody tissue. Plants which start with separate vascular bundles develop a band of meristematic cambium cells which link the vascular tissue together. These cambium cells also divide and the layer of thickening is then continuous round the stem, forming a cylinder of growing wood from which bands of cambium and parenchyma, known as medullary rays, extend into the centre of the wood. These rays serve for the lateral or radial transport of water and foodstuffs.

In spring when the supply of water and minerals from the roots is at its greatest the new xylem cells formed by the cambium are large and relatively thin-walled. As the supplies slow down in late summer, the new cells become smaller and relatively thicker-walled until finally, in autumn, growth ceases altogether. The difference in size and appearance between the last cells formed in autumn and the first in the following spring are so marked that it can be seen easily with the naked eye and give the section of the tree its characteristic ringed appearance. These annual rings of wood can tell the trained observer a great deal about the past history of a tree and the weather in previous seasons.

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