TROPOPHYTES 61 



laterally in the axil of the leaves or terminally at the end of 

 the branches. Each bud consists of a condensed shoot-axis 

 bearing leaves. On herbaceous shoots the buds develop 

 immediately into leafy shoots or flowers until exhaustion 

 puts an end to growth. The leaf-buds of trees are formed 

 in autumn and develop in the following spring. They 

 have therefore to endure the winter. To equip them for 

 this, various xerophytic characters and habits are assumed. 

 The bud is primarily protected against excessive tran- 

 spiration by the fact that a number of leaves are closely 

 packed together in a small space. In addition, they are 

 usually surrounded by corky scales, which allow no water 

 or water-vapour to pass either in or out. The escape of 

 water from within would lead to the desiccation of the 

 bud ; the entrance of liquid water from without would 

 cause it to rot, or at least make it more susceptible to 

 injury during frost. Additional security against drought 

 is provided, in some cases, by the young fohage-leaves 

 within the bud being covered with cottony hairs, and the 

 scales being closed and sealed by a secretion of gum. 

 Bud-scales, cork, cotton, and gum are, without doubt, 

 all adaptations primarily directed against drought, but 

 they benefit the buds in other ways too. They preserve 

 the tender parts within against rapid changes of tempera- 

 ture, from the ravages of injurious insects, and from 

 the attacks of disease-spreading bacteria and fungi. A 

 certain amount of protection is also afforded to buds 

 while they are immature and developing. This is secured 

 by their position in the leaf-axils, where they are, to a 

 greater or less extent, covered by the leaf-bases. In the 

 plane, the leaf-base envelops the whole bud, so that it 

 is not seen until the leaf falls ; in the willows and roses 

 the stipules are so placed as to ward off the wind ; in the 

 elder and currant {Ribes) the sheathing leaf-bases act in 

 the same way. 



The morphology of the bud-scales varies ; they are 

 always leaves or parts of leaves. In the honeysuckle, 

 privet, lUac, and holly they are complete leaves and green, 

 but somewhat modified from the ordinary foliage-leaves. 

 In the horse-chestnut, cherry, elder, maple, ash, and plum 

 they are leaf-bases only. In the sweet-chestnut, lime, 

 rose, pear, elm, blackberry, birch, oak, hazel, apple, beech, 

 poplar, wUlow, and hawthorn they are modified stipules. 



