570 



R. M. Sachs and A. Lang 



ation time in the subapical pith region of GA-treated Samolus plants 

 is approximately 24 hrs. at 26° C, 32 hrs. at 20°, and 46 hrs. at 17° 

 (Figure 4). The Qj,, for mitosis, then, is equal to 2.1, a value similar 

 to that found for the apical cells of pea roots (3). 



Approximately 6,300 pith cells are produced by GA-induced sub- 

 apical mitotic activity at 26° C. in the initial 72 hrs. (19). Assuming 

 that the pith cell generation time is 24 hrs., then there are 3 cell gen- 

 erations in 72 hrs.; hence, in the pith region in the initial 24-hr. pe- 

 riod, 900 cells, representing the first generation, divided in response to 

 GA [for calculations see (19)]. Further calculations reveal that the 

 average duration of karyokinesis (in our study equal to the time re- 

 quired for cells to progress from metaphase through telophase) is 

 about 32 min., which is very close to the figure calculated by Brown 

 (3) for the apical cells of pea roots, namely, 40 min. 



The time required for GA to diffuse to the active sites in the pith 

 cells of Samolus is less than 2 hrs. (19), which is within the limits of 

 accuracy of our determination of cell generation time. Hence, the 

 24-hr. delay in the mitotic effect of GA is an inherent trait of the 

 mechanism of action of this substance on cell division. Since the 

 delay approaches 24 hrs. and since, in addition, the pith cell gener- 



60 



/ 



GA applied 



• 9 AM 

 O 9 PM 



24 48 9 AM 



HRS AFTER APPLICATION OF GA 



9 PM 9 AM 9 PM 



TIME OF DAY 



9 AM 



9 PM 



Vi)^. 3. Periodic fliicdiations in mitotic arlivity in tlic pith tissue as related to the 

 mode of application o£ GA (A) and lime of day of llie initial application (B). In 

 the latter case, 25 fig. of GA were applied at and 24 hrs. to both groups. 



