40 THE STORY OF PLANT LIFE 



In order to resist cold, etc., the leaves may close 

 up at night or in cold weather, as in the Woodsorrel. 



The life-history of the leaf also takes into account 

 the mode of arrangement of the leaves in bud, and 

 their mode of opening or vernation. The protection 

 of these buds by stipules also is a matter of extreme 

 interest. The leaf-fall of deciduous plants is another 

 subject for study in this connection. In the case of 

 evergreen trees the surface of the leaves varies, but 

 may be connected with the prevention of an accu- 

 mulation of snow upon the upper surface in winter. 



In the case of the floral leaves and the resulting 

 fruits and seeds developed from the ovules of the 

 pistil, the adaptations which are called forth under 

 the varying conditions of existence are largely con- 

 cerned with the future generation which they play 

 such a great part in producing. Since these conditions 

 are not fulfilled without the intervention of agencies 

 other than those of the plant itself, the life-history of 

 the flower is especially interesting, in that it has to 

 do with such factors as water, wind, insects, and 

 other agencies. This applies equally to pollination 

 and to the dispersal of fruit or seed after fertilisation. 



The unfolding of the flower bud or aestivation is 

 similar in kind to that of the unfolding of the leaf- 

 bud. As a rule the aestivation of all flower buds on 

 one plant is the same. The mode of aestivation of 

 the flower buds varies in different types of plants, 

 and though they usually overlap or are twisted the 

 parts may be sometimes open. 



