522 



C. Sirowal 



C 



T 



j- 



0.5 MM. 



Fig. 1. Longitudinal sections of the external (left) and central (right) parenchyma 

 of stems of Perilla naiikinensis C, control; T, treated with 100 p.p.m. GA. 



of GA to the growing point does not promote the growth of the 

 stem, but the shape of the leaves is very strongly modified. In these 

 two cases the leaves are larger in the treated plants but their form 

 is more simple. This can only be explained by postulating a modifica- 

 tion of mitotic activity within the leaf initials. 



The action of GA on flowering of long-day (LD) plants grown in 

 short days is also an effect on cell divisions in the stem apex (24). In 

 short days the functioning of the apex is normally restricted to the 

 formation of leaf initials. The application of G\ enhances cell divi- 

 sion in such a way that the whole meristematic region is activated, 

 giving rise to the "manteau dc Gregoire" (12) from 'which the flower 

 primordium is formed. 



The occurrence of these different effects fits relatively well with 

 the anatomical and cytological description of distinct zones inside the 

 meristem as proposed by Buvat (7). We can visualize that, depending 

 on the species and upon the circumstances, GA acts selectively on one 

 or another meristematic zone of the stem and on the young tissues 

 initiated by the activity of the meristem. In stem elongation, the 

 "meristeme medullaire" and the zone situated immediately under it 

 would be activated. In modifications of leaf form, the activation 

 would affect the "anneau initial" and the leaf initials. The formation 

 of tlic flower would correspond lo a more complete activation of the 



