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 foliage-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-seales varies; they are 
always leaves or parts of leaves. In the honeysuckle, 
privet, lilac, 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, willow, and hawthorn they are modified stipules. 
