C. SIRONVALi 



Embourg, Belgium 



GibbereliLns, Cell DLVLSion, and 



Plant Flowering 



Recent studies on the gibberellins indicate that there is an activa- 

 tion of cell division as well as an activation of cell elongation (4, 14, 

 23, 24, 34). The discovery of the primary effects of gibberellin on 

 stem elongation led to the idea that the action of the gibberellins 

 was similar to that of auxin. Since that time many other activation 

 effects have been observed (6,21,22,25,26,33). Some are concerned 

 with the functioning of the growing point of the stem and particu- 

 larly with a modification of the rate or the direction of cell division. 



It is now clear that in some cases the effect of gibberellins (GA) 

 on stem elongation is partly due to enhanced cell division activity. 

 Figure 1 shows longitudinal sections of Perilla stems in which the 

 dimensions of the control cells are approximately the same as those of 

 the treated cells, while the internode length of the treated plants 

 was 2.3 times that of the control. Sometimes the stem elongation is pro- 

 moted more easily in the inflorescence than in the vegetative stem. 

 With Iberis amara, for instance, we obtained very little length in- 

 crease in the vegetative stem, but the length of the terminal inflor- 

 escence was markedly increased (Figure 2). Such specific effects have 

 been observed in Begonia (14) and in strawberry (R. Lemaitre, per- 

 sonal communication). Enhanced cell division plays an important role 

 in these effects. The activated cells are those in the zone immediately 

 under the apical meristem, as noted by Sachs and Lang (34) in vege- 

 tative plants of Hyoscyamus niger. 



Modifications of leaf form and size induced by GA have often 

 been observed. Two very characteristic cases are those of Statice 

 sinuata and Lepidmm ruderale (Figure 2). The continual application 



1 Subsequently: Laboratory of Plant Physiology, Centre de Recherches de Gorsem, 

 Gorsem-Saint-Trond, Belgium. 



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