Theof herbaceous plants are not strong enough to withstand great strains due to weight or bending, and these plants do not attain the large size reached by trees and shrubs. The rigidity of the herbaceous is partly due to the xylem of the vascular bundles, which has a mechanical as well as a conducting function. The strains to which the aerial stems of a plant are subject are due to the weight of the stems and , and also to bending caused by the wind. It is a recognized fact that when there is only a limited amount of strengthening material for use in an upright structure, the best place to put it is towards the outside in the form of a tube. In agreement with this fact the vascular bundles of a herbaceous occur in a broken ring towards the outer part of the stem.
In the stem of the young wallflower the xylem comprises all the lignified mechanical tissue, but in many plants lignified tissue is found in parts other than the vascular bundles. This mechanical tissue is termed sclcrenchyma, and generally takes the form of long pointed cells with lignified walls similar to the wood fibres of the xylem. There are many different ways in which these fibres are arranged. In some plants they occur in the cortex within the angles of the stem ; in others there is a continuous ring of fibres in the outer cortex, while in many plants patches of fibrous tissue occur on the outer side of each vascular bundle. Collenchyma l is common in the outer parts of the stems of many herbaceous plants and is made up of living cells with cellulose walls which are thickened at the angles where several cells meet. This thickening of the walls of the cells gives them a mechanical function.
The lignified tissue and collenchyma play their part in the maintenance of the rigidity of the herbaceous stem, but they are helped in this by the parenchymatous cells of the cortex and pith. When such living cells are stretched and swollen by pressure due to fluid in them, they are said to be turgid or in a state of turgor. Owing to the turgor of the cells, the pith of a herbaceous stem tends to become longer, but this stretching is prevented by the mechanical tissues of the outer part of the stem. This state of tension between the inner and outer tissues of the stem helps to keep it firm and rigid. It is easy to demonstrate the existence of tension between the inner and outer tissue of the stalk of a dandelion flower by splitting it longitudinally into four strips ; each strip immediately becomes curved so that the pith is on the convex side. The splitting releases the tension and the pith then stretches, while the outer part of the stem which has been partly stretched by the pith contracts slightly.
The Growth in Thickness of Trees and Shrubs
The aerial stems of trees and shrubs, unlike those of most perennial, do not die down at the beginning of winter but persist all the year round, and during their lifetime show both an increase in height and in the number of their branches. At the same time the stems also grow in thickness ; it is essential that this should take place for three reasons : Firstly, the greater size of the branches and twigs, and also the greater number of that are borne by the new stems, cause the main stem to carry a bigger weight. Secondly, the extra leaves necessitate an increased amount of conducting tissue in the stem. More xylem is needed to conduct water to the leaves, and more phloem is necessary to carry the increased quantity of foodstuffs manufactured by the leaves to other parts of the plant. Thirdly, new conducting tissue is required to carry on the work of the older xylem and phloem which have ceased to function.
The new wood of shrubs and trees formed during the increase in thickness is the secondary xylcm, which is formed by the activity of the cambium and consists of vessels, tracheids and medullary rays. If a transverse section of a tree trunk is examined, the wood is seen to be marked by a number of concentric rings. These are calledrings, for each usually represents a year ’s growth. This ringed appearance of the wood is due to the different texture of the xylem formed at different times in the year. In winter the formation of secondary xylem ceases, so that the autumn wood lies next to that formed in the spring. The xylem cells formed in the autumn are small and have thick walls, but those formed in spring are larger and have thinner walls. The rings are thus caused by the alternation of layers of small-celled autumn xylem with layers of large-celled spring xylem. The different sizes of the cells of the spring and autumn wood is probably due chiefly to the different water requirements of the shoots at these seasons. In spring more water is required by the shoots for the expanding leaves than in autumn when the leaves have fallen. The fact that one annual ring is usually formed per year enables the age of a tree trunk or branch to be found out by counting the total number of rings. Occasionally two or more rings are formed in one year. This may be due to the destruction of the young leaves by frost or insects, causing the dormant buds to develop and another crop of leaves to be grown.
Heart and Sap Wood
As a tree gets older and its trunk increases in girth, the wood at the centre of the trunk, termed the heart wood, ceases to conduct water. This function is then carried out only by the outer wood or sap wood. The conducting cells of the heart wood become blocked up and the parenchymatous cells die. In some trees, for example the willow, the heart wood is the same colour as the sap wood, and gradually rots away, thus leaving the trunk hollow. In most trees, however, the heart wood is darker in colour than the sap wood, and being also harder and stronger it does not decay.
The Secondary Phloem
Although the cambium produces secondary phloem as well as secondary xylem, the space occupied by the phloem in a woody stem is small compared with that taken up by the xylem. This is partly because the phloem, being soft-walled tissue, is crushed by the growing xylem, and partly because during the division of the cambium more xylem than phloem is produced. In the lime the secondary phloem consists of bands of sieve tubes alternating with bands of lignified fibres.
Cork and Bark
As the young stem of a plant increases in thickness owing to the activity of the cambium, the epidermis does not make allowance for the growth by expansion or cell division, and consequently splits develop in it. The exposure of the cortical tissues is prevented, however, by the formation of cork. The tissue consists of narrow, thin-walled, empty cells which are arranged in regular rows. The cells are produced by the division of a cambium which usually develops in the layer of cortex beneath the epidermis, but is sometimes formed deeper in the cortex. In the wallflower stem the cork cambium develops in the pericycle. The cambial cells divide tangentially and cells are cut off both inside and outside the ring. Those cut off on the outside gradually lose their contents, and their walls become impregnated with a substance known as suberin, which is impermeable to water. The cells cut off on the inside of the cambium retain their living contents and cellulose walls and become added to the cortex. Generally the first cambium does not remain active for very long, but is replaced by one formed deeper in the cortex, and this is superseded by others until the cambium is formed close to or in the phloem. As the cork cells accumulate they form a watertight covering for the stem, and the epidermis and cortical tissue which lies outside the cortex eventually die through being cut off from the water supply. This dead tissue, together with the cork, forms the bark of the plant. As the thickness of the stem increases, the bark becomes stretched and marked by longitudinal splits and fissures.
Cork formation also occurs in the older parts of the. The cambium is always formed in the pericycle, so that all the cortex is cut off and becomes bark.
When the epidermis of the stem has been replaced by cork, which is impermeable to gases, air can no longer enter the tissues of the stem by the stomata of the epidermis. The aeration of the stem is then effected by larger breathing pores called lenticels. At certain places in the cork cambium the cork cells produced are not close-fitting and brick-shaped, but are rounded and fit loosely together. These powdery cork cells eventually burst open the outer tissues of the stem, and then air is able to penetrate into the stem through the intercellular spaces between them. The lenticels appear as minute light-brown patches on the surface of the stem and are clearly visible to the naked eye. In winter the lenticels are closed by the development of a layer of the usual close-fitting cork cells beneath the powdery cork cells of the lenticcl.