EXTERNAL INFLUENCES AS CAUSES OF VARIATION 237 



accordance with this inward polarity and the new conditions 

 (see Figs. 25 and 26). 



Vochting's experiments as here figured, and as described by 

 Morgan, 1 indicate that the formation of stem or of root is a 



E 



FIG. 26. Influence of polarity and of gravity upon the character and direction of 

 growth. After Morgan, from Vochting 



A, piece of willow (cut off in July) suspended in moist atmosphere, with apex upward; B, 

 older piece of willow (cut off in March) suspended in moist atmosphere, with apex 

 downward ; C, piece of willow with a ring removed from the middle,' apex upward ; 

 D, piece of root of Pofulus dilatata, with basal end upward; shoots from basal 

 callus; E, piece of root of same with two rings removed; new shoots develop from 

 basal callus and from basal end of each ring 



question of both internal causes (polarity) and external influ- 

 ences (gravity), that is to say, the character of growth is a func- 

 tion both of internal and external conditions. 



1 Morgan, Regeneration, pp. 72-8^ 



