100 



The Unity of the Organism 



be described as the bi-gradient, the direct gradient and the 

 reverse gradient types, depending on whether the gradation 

 is from the mid-region of the axis both ways (figure 58), from 

 the proximal toward the distal end (figure 59), or from the 

 distal toward the proximal end (figure 60). 



FIGURE 59. VICIA OIOAKTEA. 



FIGURE 60. CASSIA SP. 



Almost all simple leaves of seed plants show something of 

 the same scheme. As examples, typical elliptical-entire 

 leaves of the elm and poplar and such typical lobed leaves as 

 those of most oaks may be pointed to. 



Nearly every twig of a tree which represents a single 

 annual growth impulse, in cases where the growing period is 

 restricted to a small part of each year, presents a size gra- 

 dient in the leaves distributed along the axis. A particularly 

 striking illustration of this is furnished by the California 



