Regeneration 165 



away from the leaf will occur. This will allow one or 

 more of the notch buds of this leaf to grow out and then 

 a flow will be established towards these growing buds. 



In the third specimen, Fig. 18, the presence of two 

 leaves suppresses or, as a rule, retards the growth of a 

 shoot on the stem and possibly also the flow from one 

 leaf may block to some extent the flow from the op- 

 posite leaf if the piece of stem is very short. This puts 

 the leaves in a condition not as good as that in leaf Fig. 

 20, but better than in leaf Fig. 19. * 



In the nprmal plant the buds in the notches of the 

 leaf remain dormant since the flow of the "stimulating" 

 substances takes place towards the tips of the stem and 

 root, and because these substances are retained there in 

 excess. This is probably the real basis of the mysterious 

 dominance of the "whole" over its "parts" or of the 

 anlagen of the tip of the stem over those farther below. 

 When a piece of the stem of Bryophyllum is cut off and 

 its leaves are removed, the two apical buds will grow 

 out first. This "dominance" finds its explanation 

 probably in the anatomical structure and the mechan- 

 ism of sap flow which tend to bring the "stimulating" 

 substances first to the anlagen in the tip. In Laminaria 

 Setchell has been able to show directly that regenera- 

 tion always starts from that tissue which conducts the 

 nutritive material. 



When we cut out a piece of a stem of Bryophyllum, 



1 With larger leaves the experiment may also succeed in moist air. 



