Borthwick et al. —73— Wave Length Dependence 



was not possible in experiments with filtered radiation to employ narrow 

 and sharply limited bands and at the same time to retain adequate energy. 

 A further source of difference in results probably arose from the type of 

 biological response measured. In most cases the various investigators 

 employed the time required for the production of flowers as a measure of 

 effectiveness of a given treatment. The process of flowering is usually of 

 several weeks' duration, from the time of initiation to the final opening of 

 flowers. It consists of a number of rather distinct phases each of which 

 in different plants may or may not be similarly influenced by a given photo- 

 periodic treatment (5). Different results, therefore, could have been 

 obtained from different plants. In the work of this laboratory the biologi- 

 cal response measured has been limited to floral initiation and all of the 

 results have been obtained from dissections of the experimental plants 

 within a week after conclusion of the treatments. By means of the spectro- 

 graph narrow wave length bands of very pure radiation have been obtained 

 and with a carbon arc adequate energy has been available throughout the 

 visible. 



Experimental evidence for the basic theory that flowering is controlled 

 by a hormone-like material produced in the leaf is given in papers cited 

 by MuRNEEK (13). Although Lang and Melchers' observation (9) 

 that flowering in Hyoscyamus niger can be induced by defoliation may 

 suggest that the leaf is not the locus of the photoperiodic reaction, a quite 

 different interpretation is advanced below which reconciles their results 

 with other data. Evidence that flowering is regulated by means of a 

 hormone-like material, however, is hypothetical and will probably remain 

 so until some type of assay is devised. 



The reactions that cause floral initiation in short-day plants might at 

 first appear unrelated to those that cause initiation in long-day ones. Sev- 

 eral workers, however, have been led by their experiments to advance the 

 idea that both types of plants are similar in underlying processes of initia- 

 tion. Thus WiTHROW and Withrow (25) state: "The photo-activated 

 molecule involved in photoperiodism is probably the same for both long- 

 day and short-day plants since the same general regions of radiation are 

 effective in producing their respective responses." Funke (6) also recog- 

 nized a basic similarity of the two types of plants in their response to dif- 

 ferent colors of light. This essentially is the concept back of Cajlachjan's 

 (3) introduction of ihe term "florigen" for the "flower forming hormone" 

 although an interrelationship of long-day and short-day plants is not shown 

 by his work. Melchers (10) induced a short-day plant, Maryland Mam- 

 moth tobacco, to flower on long-day by grafting a leaf of a long-day plant, 

 annual Hyoscyamus niger, to it and thus showed that material derived from 

 the latter could bring about flowering of the former. Cholodny (4) was 

 careful to emphasize that this type of result indicated that a material con- 

 trolling flowering in one plant is effective in another even of different 

 genus but did not require the flower inducing materials to be identical. 



The concept that long-day and short-day plants are basically similar 

 in photoperiodic response is strongly supported by other experiments that 

 have been described and by current work in this laboratory. The action 

 spectra obtained in the latter work give a quantitative expression to results 

 that were qualitatively anticipated by Katunskij (7) and by Kleshnin 



