416 PLANT PHOTOPERIODISM 



light be replaced by applied sugar? Does it require CO-? What kinds of 

 inhibitors inhibit light stabilization? Alas! it is easy to make sugges- 

 tions for further experiments. 



The transport of flowering stimulus from the leaf appears to a 

 detectable extent approximately 2 to 4 hr after the end of the inductive 

 dark period. In the experiments of Salisbury (1955) the half-time for 

 transport, the time required to accomplish one-half of all translocation 

 which will ultimately take place, is approximately 16 hr after the end 

 of the inductive dark period. After a further 28 hr or so, or approxi- 

 mately 44 hr after the beginning of the dark period, the transport of 

 flowering stimulus from the leaf asymptotically approaches comple- 

 tion. It is apparent at once that transport of the flowering stimulus 

 from the leaf does not take place to a detectable extent until a fair 

 proportion of the high-intensity light stabilization has been consum- 

 mated. This process is apparently necessary for effective transport of 

 the flowering stimulus from the leaf. It is not absolutely essential, how- 

 ever, since, as is well known, Xanthium flowers in response to an in- 

 ductive dark period even if left in continuous dark. In the second place 

 it appears probable that the transport of flowering stimulus from the 

 leaf is slow as compared to the transport of other materials from 

 leaves. Thus, for example, in the work of Burrows (1957), the trans- 

 location of labeled carbohydrates from leaves is found to be consider- 

 ably more rapid. Leaves were allowed to label themselves with C^^02 

 for 8 min. At the end of the 8-min labeling period the plants were re- 

 turned to unlabeled COl> and allowed to continue photosynthesis. The 

 half-time for sweeping of the labeled sugar from the leaf was found to 

 be approximately 1 hr. In similar experiments the half-time for re- 

 moval of 2,4-D from the treated leaf was found to be approximately 

 3 hr. It is apparent therefore that the flowering stimulus is not rapidly 

 transported or possibly is not rapidly absorbed into the transport sys- 

 tem as compared to the usual readily transported metabolites. 



From an operational point of view the property of transportability 

 within the plant is the most accessible and most characteristic property 

 of the flowering stimulus. It should perhaps be this property more than 

 any other which might be expected to aid in identification of the ma- 

 terial. Let us plan an experiment in which the property of transporta- 

 bility might be used to advantage. We might, for example, allow the 



