STEMS 



739 



Amorpha canescens, which are sometimes one and sometimes the other, 

 might throw light upon the problem. 



The advantages of variation in stem form. The capacity for dif- 

 ferential elongation possessed by aquatic stems and by many aerial 

 stems when submerged by sand is of obvious advantage in that the lift- 

 ing of the leaves into the light and the air is thus made possible. Of 

 unusual significance is stem dwarfness, as illustrated by cushion plants 

 and by the Krummholz, since such habits are admirably suited for pro- 

 tection, especially from excessive transpiration. Such protection is due 

 in large part to the reduced surface exposure resulting from the compact 



a 



1062 



FIGS. 1061, 1062. Spinose branches: 1061 A, a spinose branch of the wild crab 

 apple (Pyrus coronaria) in its first year; note that the terminal bud soon ceased to develop; 

 1061 B, a similar branch in its second year, showing a lateral bud that continued to develop ; 

 5, leaf scar indicating the position of a leaf of the first season ; 1062, a compound spi- 

 nose branch of a hawthorn (Crataegus punctata), showing that such spines are branches 

 whose terminal buds soon cease development; note that leaves occur on the lateral 

 branches (t), as well as on the main axis (a). 



