Growth Substances 



395 



factor coming from the cotyledons. Pilet (1952) observed that Semper- 

 vivum leaves parasitized by Endophyllum sempervdvi contain much 

 more auxin than normal ones and, presumably for this reason, are accel- 

 erated in their development. The effectiveness of adenine in leaf formation 

 has been observed by various workers (D. Bonner and Haagen-Smit, 

 1939). Auxin does not seem to promote the growth of the blade as a whole, 



V 



Fig. 18-18. Section through leaf blades 

 of Kalanchoe Blossfeldiana. At left, from 

 a plant grown under long days. At right, 

 from one grown under short days. The 

 difference in thickness is entirely due to 

 cell size. ( From Harder and von Witsch. ) 



however, although when embryonic leaves are treated with it, various 

 changes may be produced (Laibach and Fischnich, 1936; Zimmerman, 

 1951k; Applegate and Hamner, 1957). These are probably to be looked 

 upon as injuries rather than formative effects. Wenck (1952) has studied 

 the stimulatory and inhibitory effects of auxin and of various auxin 

 antagonists on leaf growth in a number of species. 



