The twig of the sycamore shows the characteristic features of twigs ; it is greenish-grey or light brown in colour, and is marked by many small light-brown patches, which are the lenticels or breathing pores. At the tip of the twig there is a large apical bud, beneath which is a pair of lateral buds. Other pairs of lateral buds occur farther down theand each pair is arranged at right angles to the pair above and below it, I.e. the bud arrangement is decussate. Some twigs have no apical buds and are the shoots which have borne a terminal inflorescence in the previous year. The apical growth of the twig ceases with the development of the inflorescence. Beneath each bud is a light-coloured, V-shaped scar marked with four or five small spots. The scar is the mark left when the drops off in the autumn, and the small spots show where the vascular bundles passed from the stem to the stalk. Examination of the stem shows that each bud arises in the axil of a leaf. The buds are covered by green bud scales which overlap each other regularly and protect the stem tip from insects, frost, mechanical injury and the effect of drying winds. If the scales are removed carefully the inner ones are seen to be lined with short hairs which give them a silvery appearance. The stem apex, which lies within the bud scales, is a small blunt projection covered by two or three pairs of minute foliage , each of which is folded up like a fan. The larger terminal buds usually contain small undeveloped as well as .
The Bursting of the Buds
In spring the stem apex begins to lengthen, and the bud swells owing to the growth of the leaves within it. The bud scales are pushed apart and eventually drop off, leaving a number of bud scale scars which form a ring round the stem. These rings of scars occur at intervals along the twigs, and since the stem between two consecutive rings represents one year ’s growth it is possible to tell the age of a twig by counting the number of the rings. In time the bud scale scars become obliterated by the growth in thickness of the stem, and cannot be recognized on the older branches. As the stem lengthens, the leaves unfold and new leaves develop at the apex of the twig. At the same time some of the lateral buds of the last season ’s growth of stem commence to form side shoots. Other lateral buds do not sprout but remain dormant and only develop if the apical bud is injured or cut off by. Such dormant buds remain capable of giving rise to shoots for many years before they eventually die. In the case of the large horizontal branches, the lateral buds that occur above and below the stem remain dormant, and only those at the side grow into branches. The chief determining factor in the development of side shoots from lateral buds is light, for if every lateral bud gave rise to a leafy shoot, overcrowding and shading of the leaves would result.
Occasionally some of the inner scales of a sycamore bud bear at the tip a short green tuft which shows division into four or five lobes, and is a rudimentary leaf blade. The occurrence of these rudiments of the leaf blade suggests that the bud scales represent modified leaf bases.
Similar traces of the leaf blade are found at the top of the inner scales of the horse chestnut bud. If a beech bud is dissected it will be found that the bud scales are arranged in pairs, and most of the small foliage leaves within the bud have a pair of large scale-like stipules attached at the base. These stipules remain on the leaves for two or three weeks after they have become fully expanded, and then fall off. In the beech, therefore, it appears that the outer paired bud scales represent modified stipules. The bud scales of the oak are likewise modified stipules. The gradual transition from bud scales to foliage leaves is well shown if the bud of the bird cherry or of the flowering currant is dissected.
The Fall of the Leaf
In the winter the soil becomes cold, and although there may be plenty of water in it the absorptive power of thedecreases owing to the low temperature. Since the water uptake by the is therefore diminished, it is essential that the evaporation of water from the leaves should also be cut down, and in the case of deciduous trees this is effected by the fall of the leaves. In the autumn any food substances remaining in the leaves are transported to the storage cells of the stem. The chlorophyll becomes broken up and the useful products formed pass into the stem, leaving behind the yellow constituents of the pigment. These yellow pigments, together with a red pigment which collects in the dying leaf, are the cause of the yellow, brown and red tints of the autumn leaf.
The leaf fall is not merely due to the death of the leaf but is the result of activity on the part of the plant itself. At the base of the leaf stalk certain cells separate and become rounded off from each other ; these cells form the absciss layer. Along this layer the leaf stalk gradually loosens and finally breaks off from the stem. The cells of the vascular bundles do not separate but become broken through, and the cell cavity is either crushed or becomes filled up with resinous excretions from the surrounding cells. The exposed surface left on the stem after the leaf has fallen remains protected by the development of a layer of cork beneath the cells of the absciss layer. This cork prevents the evaporation of water from the scar and also stops the entrance of parasitic fungi.
THE HORSE CHESTNUT TWIG
This twig in its general structure and method of branching resembles the sycamore twig but it is much stouter. Its buds are larger with brown, sticky bud scales and all the scars are more prominent.