Double Adaptations 455 



thelier with Berheris, Lycium and other species, 

 which lose their spines in damp air. 



In some conifers the evolution of horizontal 

 branches may be modified by simply turning 

 the buds upside down. Or the lateral branches 

 can be induced to become erect stems by 

 cutting off the normal summit of a tree. Nu- 

 merous organs and functions lie dormant un- 

 til aroused by external agencies, and many 

 other cases could be cited, showing the wide 

 occurrence of double adaptation. 



There are, however, two points, which should 

 not be passed over without some mention. One 

 of them is the influence of sun and shade on 

 leaves, and the other the atavistic forms, often 

 exhibited during the juvenile period. 



The leaves of many plants, and especially 

 those of some shrubs and trees, have the ca- 

 pacity of adapting themselves either to intense 

 or to diffuse light. On the circumference of the 

 crown of a tree the light is stronger and the 

 leaves are small and thick, with a dense tissue. 

 In the inner parts of the crown the light is 

 weak and the leaves are broader in order to 

 get as much of it as possible. They become 

 larger but thinner, consisting often of a small 

 number of cell layers. The definitive forma- 

 tion is made in extreme youth, often even 

 during the previous summer, at the time of the 



