244 



PLANT RELATIONS. 



certain period, is an adaptation to climate. In the tem- 

 perate regions the adaptation is in response to the winter 

 cold, when a vast reduction of delicate exposed surface is 

 necessary. Instead of protecting delicate leaf structures 

 from the severe cold of winter, these plants have formed 

 the habit of dropping them and putting out new leaves 

 when the favorable season returns. 



It is instructive to notice how differently the conifers 

 (pines, etc.) and the deciduous trees (oaks, maples, etc.) have 



answered the problem of adaptation 

 to the cold of winter. The conifers 

 have protected their leaves, giving 

 them a small surface and heavy 

 walls. In this way protection has 

 been secured at the expense of 

 working power during the season 

 of work. Eeduced surface and 

 thick walls are both obstacles to 

 leaf work. On the other hand, 

 the deciduous trees have devel- 

 oped the working power of their 

 leaves to the greatest extent, giving 

 them large surface exposure and 

 comparatively delicate walls. It 

 is out of the question to protect 

 such an amount of surface during 

 the winter, and hence the decidu- 

 ous habit. The conifers are saved 

 the annual renewal of leaves, but 

 lose in working power; the de- 

 ciduous trees must renew their leaves annually, but gain 

 greatly in working power. 



It should be remarked that leaves do not fall because 

 they are broken off, but that in a certain sense it is a 

 process of growing off. Often at the base of the leaves, 

 where the separation is to occur, a cleavage region is 



Fig. 208. A section through the 

 base of a leaf of horse-chestnut 

 preparing to fall off at the end 

 of the growing season. A 

 cleavage plate (s) has devel- 

 oped between the woody bun- 

 dle (b) and the surface. Pres- 

 ently this reaches the surface, 

 and only the woody strand 

 fastens the leaf to the stem. 



