524 C. Sironval 



mitotic capacity of the meristem as a whole including the "meristeme 

 d'attente." These three effects could coincide in time. 



To distinguish between the different meristematic activities of the 

 growing point of the stem, one can compare the differential action of 

 GA on LD and SD plants, \\lien GA is applied to SD plants grown 

 under long days, it cannot induce flowering but acts only on stem 

 elongation. When applied to LD plants grown in short days, how- 

 ever, GA promotes both stem elongation and flowering. Using Bu- 

 vat's concept, this means that the "meristeme d'attente" cannot be ac- 

 tivated by GA in SD plants, while it can be activated in LD plants. 

 Tschailachjan (42) has proposed a theory that accounts for this. He 

 postulates that florigen is composed of two hormones, GA and an- 

 thesin. When the two hormones are both present, flowering is pro- 

 moted, as evidenced by the flowering behavior of SD plants under 

 short days and LD plants under long days. Following Tschadachjan, 

 LD plants synthesize anthesin in short days, and the addition of GA 

 results in the formation of florigen (GA -|- anthesin). On the other 

 hand, SD plants synthesize GA in long days, and a further addition 

 of GA has no effect on flowering because anthesin is lacking. It is 

 clear that these differences in reaction when GA is applied indicate 

 that an SD plant grown under long days is not identical to an LD 

 plant in short days. There is a sort of dissymmetry which Tschailach- 

 jan's proposal attempts to interpret. 



But this does not change the principal open question: How does 

 GA activate cell division in the young tissues of the stem? 



BIOCHEMICAL APPROACH TO THE ACTIVATION OF 



CELL DIVISION BY GA 



Biochemically speaking, cell division is a very complicated phe- 

 nomenon involving the synthesis of protein for which several bio- 

 chemical conditions must be met. The role of ribonucleic acids in 

 protein synthesis has been shown in animals as well as in plants. 

 Protein synthesis also depends on the availability of a suffic ient source 

 of energy w^ith the resulting adenosine triphosphate (ATP) playing a 

 prominent role (1,3, 8). This ATP may be synthesized in both respira- 

 tion and in photosynthesis (16, 43). It is likely that some of these con- 

 ditions are absent in meristems, particularly in the "meristeme d'at- 

 tente" of an LD plant grown in short days or in that of an SD plant 

 grown in long days. Protein synthesis would, therefore, be at a level 

 insufficient for accelerated cell division. The type of block may well 

 be different for the two groups of plants. 



It is highly probable that GA can modify the rate of protein syn- 

 thesis in the cells of the growing point of the stem. Further promotion 

 of resj^iration (2, 19), action on several cn/yme systems (40, 41), modifi- 



