240 PLANT RELATIONS. 



well-known thicket plants, such as hazel, birch, alder, etc. 

 Although pure thickets frequently occur, that is, thickets 

 in which willow, or hazel, or alder, is the prevailing type, 

 mixed thickets are probably more common. One who is 

 familiar with mesophyte thickets will recognize that the 

 ordinary mixed thickets are composed of various kinds of 

 shrubs, brambles, and tall herbs. 



167. Deciduous forests. Deciduous forests are especially 

 characteristic of temperate regions. The deciduous habit, 

 that is, the habit of shedding leaves at a certain period,, is 

 an adaptation to climate. In the temperate 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 pro- 

 tection has been secured at the expense of Working power 

 daring the season of work. Reduced surface and thick 

 walls are both obstacles to leaf work. On the other hand, 

 the deciduous trees have developed the working power of 

 their leaves to the greatest extent, giving them large sur- 

 face exposure and comparatively delicate Avails. It is out 

 of the question to protect such an amount of surface dur- 

 ing the winter, and hence the deciduous habit. The coni- 

 fers are saved the annual renewal of leaves, but lose in 

 working power ; the deciduous 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, 



