76 • Floral Hormones and the Induced State 



however, it is also consistent with the idea that the flowering 

 hormone might be taken up by the noninduced tissue and de- 

 stroyed by it. 



The latter interpretation is also suggested by analogous experi- 

 ments in which parts of a single leaf are subjected to long-day or 

 short-day treatments. If the basal part of the leaf is given short 

 days and the apical long days, flowering occurs, but if the situation 

 is reversed, the flowering is weak or absent. This is not due to the 

 inability of the apical portion to respond to short days and lead 

 to flowering, since it does so if the entire basal part is trimmed off 

 as long as the vascular connection to the stem is left intact. Here 

 again, noninduced tissue evidently inhibits flowering when it is 

 situated between induced tissue and the growing point, and possibly 

 does so by destroying the floral stimulus. Earlier experiments by 

 Chailakhyan with Perilla leaves also lead to the same conclusion 

 (seeNaylor, 1953). 



The most thorough recent studies of the interactions of various 

 parts of the plant on the effectiveness of localized inducing treat- 

 ments are those of Lincoln, Raven, and Hamner (1956, 1958), using 

 Xanthium. The first paper bears most directly on translocation. 

 With two-branched plants, the intensity of flowering in the re- 

 ceptor branch (long days) is inversely proportional to the amount 

 of mature tissue left on it. If, however, a carbohydrate deficiency 

 is produced in the receptor by heavy shade, the inhibition by the 

 long-day leaves is greatly reduced. Conversely, shading the donor 

 (short-day) branch, which would produce a carbohydrate deficit in 

 it, reduces flowering in the receptor. So also does removing the 

 receptor's young leaves, which are responsible for a great portion 

 of its carbohydrate uptake. Although these results are consistent 

 with the carbohydrate-flow hypothesis, several others suggest a 

 more complex situation. The inhibiting effect of mature leaves on 

 the receptor is not simply proportional to the amount of light they 

 receive but depends on its timing; that is, the effect is photo- 

 periodic. For example, the inhibition caused by leaves given 12 

 hours light-12 hours dark cycles is much greater if each night is 

 interrupted by three evenly spaced 10-minute light-breaks than if 

 interrupted only once, in the middle, by a 30-minute light-break. 

 If only carbohydrate production were involved in the inhibition, 

 such results would not be expected. 



