Shoot Histogenesis and the Subnpical Meristem 



571 



60 



24 48 



HRS. AFTER APPLICATION OF GA. 



72 



Fig. 4. The effect of temperature upon pith cell generation time. Each point is the 

 average count made for six plants. The arrows indicate the point from which the 

 generation times have been measured. All plants received 100 /xg. GA at hrs. 



ation time is also 24 hrs., it appears that the stage at which GA 

 exerts its first effect is early interphase (12, 19). 



With regard to shoot growth in rosette plants, there is little doubt 

 that the subapical meristem is responsible for the marked increase in 

 stem elongation following GA treatment. In fact, the cell contribu- 

 tion from this region is so large that the direct contribution of the 

 apical meristem to GA-induced stem growth in rosette plants may be 

 disregarded (18, 19). Furthermore, at least two facts show that GA- 

 induced stem elongation is closely related to the natural case in rosette 

 plants: (A) A gibberellin-like substance extracted from the fruit of 

 wild cucumber (15) initiates the same cytohistological development as 

 GA (18) and, eventually, flowering (13). (B) In rosette plants in which 

 stem elongation is induced by the proper environmental conditions, 

 the subapical meristem develops in the same manner as in the GA- 

 treated plants, although more slowly. 



