568 R. M. Sachs and A. Lang 



Table 1. Length of pith cells oi Samolus following application of gibberellic acid. 



Numbers in parentheses indicate range in ceil length for each zone. 



application. It is clear then that in the initial stages of stem elonga- 

 tion GA causes only an increase in cell number. Equally noteworthy 

 is the fact that more than 80 per cent of the induced cell divisions 

 are transverse; i.e., they are oriented to contribute to stem elonga- 

 tion (18,20). 



What are the quantitative aspects of GA-induced subapical meri- 

 stematic activity? In a few experiments with Samolus in which the 

 dose was varied, a minimum cell division response was obtained 

 with 0.5 ^g. GA per plant, but for several reasons discussed elsewhere 

 (19) it is difficult to establish the precise relationship between GA 



200y 



400,1 



1000^ 



I400y 



DISTRIBUTION OF CELL DIVISION 



I'ig. 1. The effect of GA upon stem elongation and cell division in Samolus. Num- 

 i)cr and position of mitotic figures per -18 ^ median longiluilinal section at 20° C. 

 There were six plants per group collected at the indicated limes, and tiic diagrams 

 are composites of six median sections (each 8 fi tliick) taken from one plant. Each 

 dot represents a mitotic figure. The boundaries of the vascular tissue and apical 

 meristem are inilicated hv dasiied lines. 



