Shoot Histogenesis and the Subapical Meristem 



569 



dose and niilodc activity. We have, however, studied in some detail 

 the nature of the cell division res[)onse in Samolus at saturating 

 doses of GA (in excess of 10 /i.g. per plant daily). With these rela- 

 tively large doses, periodic (Uictuations in the number of mitotic fig- 

 ures appearing in the pith tissue could be observed for at least 72 hrs. 

 (I'igure 2). I'hese maxima are unrelated to the time of day or manner 

 of a])})lic:ation of GA (Figure 3). They are, however, related to the 

 temperature at which the treated plants were maintained and the 

 initial application of GA, always appearing at definite times after the 

 initial application, this time depending on temperature (Figure 4). 

 Thus, we concluded that the peaks in mitotic counts reflect a partial 

 synchronization of (cll division and ihat the average pith cell gener- 



U 



— I 

 CO 



LU 



=) 



O 



u 



I— 

 O 



Si 



HRS. AFTER APPLICATION OF GA. 



Fig. 2. Counts ot uansverse mitotic figures in the pith tissue at various times after 

 the initial application of GA at 26° C. Each point is the average of counts made for 

 six plants. The plants received 25 yug. of GA at 0, 21, and 48 hrs. 



