THE LEAF 



69 



organisms. Prior to the fall of the leaf its tissues are depleted of all 

 useful materials, which are transferred down the petiole to the axis. 

 This is accompanied by changes in the cell-contents which give the 

 varied autumnal tints. What falls away is then little more than an 

 empty skeleton. Its removal leads to a great reduction of the exposed 

 surface of the plant, with the result that there is less loss from evapora- 

 tion; and less resistance to the winter winds. As a whole the plant 

 enters a dormant condition in autumn, 

 partly determined by the climatic 

 conditions, partly by the absence of 

 those organs which play so active a 

 part in its vegetation. But the fall 

 of the leaf is not an inherent feature 

 in any group of plants, nor does the 

 abciss layer form any constant limit 

 between leaf and stem. For example, 

 the British Oak (Quercus robur) is 

 deciduous, that is, it drops its leaves in 

 autumn ; but the Holm Oak (Quercus 

 Ilex), which is a native of the Mediter- 

 ranean region, remains evergreen. The 

 common Cherry (Prunus cerasus) drops 

 its leaves in autumn ; but the Cherry 

 Laurel (Prunus lauro-cerasus) is ever- 

 green. Thus though the leaf-fall is a 

 very striking feature of many trees 

 and shrubs in temperate climates, it 

 is really nothing more than a seasonal, 

 and often a specific, adaptation. In 

 many woody plants it does not occur 

 at any regular intervals, while in most 

 herbaceous plants, and especially in 

 annuals such as the Sunflower or Bean, the whole shoot simul- 

 taneously ceases its vegetative activity, leaf and axis remaining 

 connected till they rot. 



From the description which has been given of the structure of the 

 leaf it will be seen how well that organ is fitted for carrying out the 

 duty of nutrition, while exposed to the ordinary climatic conditions. 

 In the first place a broad expanse of green tissue is required, so as to 

 intercept the light. The larger its area the better. This is a sufficient 

 reason for the existence of the wide-spread blade. The active cells 



FIG. 52. 



Vertical section through the base of a 

 petiole (pet) of a Horse-chestnut at its 

 junction with the axis, showing the abciss 

 layer (ab) with cork (ck) beneath it. (/) = a 

 lenticel. When the leaf falls the scar is 

 protected, and the axillary bud (ax. b) is 

 left attached. F. O. B. (xio.) 



