226 CHEMICAL AGENTS AND GROWTH 



sented in the discussion which follows. It will attempt ( 1 ) to define the 

 organ which is responsible for setting the dormancy mechanism into 

 motion, (2) to find a lead pointing toward the kind of substance one 

 should look for, and ( 3 ) to present preliminary studies of the fluctua- 

 tions caused by photoperiodic treatments, in the levels of endogenous 

 auxins and inhibitors. 



RECEPTOR ORGAN 



In many cases, the receptor organ for the photoperiodic stimulus has 

 been found to be the leaf. Such is the case in the induction of flowering 

 in Xanthium (Hamner and Bonner, 1938) and in the control of vege- 

 tative growth in Weigela (Downs and Borthwick, 1956). The follow- 

 ing experiments, taken from the work done in this laboratory by Wax- 

 man (1957), show conclusively that the photoperiod regulates the 

 amount of growth produced by terminal or axillary buds through the 

 intermediary of certain well-defined leaves. 



Induction of Growth Inhibition through Leaves 



Let us consider plants of Cornus florida var. rubra growing under 

 long days. If we decapitate a vigorously growing branch, the buds 

 existing in the axil of the two top (opposite) leaves will start to de- 

 velop and produce new shoots (Fig. lA). If we subject to short days 

 one of the two top leaves, by placing a lightproof envelope over that 

 leaf every day from 5 p.m. until 8 a.m. (starting on the day the decapi- 



SD SD SD 



^4^^^!^^^jk^ 



B 



LONG DAYS 



SHORT DAYS 



Fig. 1. Decapitated branches of Cornus florida rubra kept under long, 

 18-hr days (A, B, C) or under short, 12-hr days (D). The development of 

 the top axillary shoots is completely prevented when the whole plant is 

 placed under short days and partially prevented when only one or two of 

 the uppermost leaves are subjected to short days (adapted from Waxman, 

 1957). 



