496 RADIATION BIOLOGY 



CORRELATION OF SOME RESPONSES TO LIGHT 



Whereas knowledge about the initial dark and light reactions is limited 

 to some physical aspects, even less is known about the subsequent reac- 

 tions in plants which are physiologically and morphologically expressed 

 as flowering, bulb and tuber formation, stem elongation, and abscission. 

 Since the effective materials are formed in the leaves and bring about 

 changes elsewhere in plants, they well might be hormones. In particular, 

 one material called "florigen" (Cajlachjan, 1937) has been postulated as 

 controlling flowering. Hormones other than auxin which have been 

 suggested by physiological responses are "vernalin" (Melchers, 1939; 

 Melchers and Lang, 1941), "metaplasin" (Harder and Bode, 1943), and 

 "flowering substance" (Struckmeyer, 1950). Vernalin was postulated to 

 account for induced blooming of biennial Hyoscyamus plants in their first 

 year. Its presence was later considered (Melchers and Lang, 1941) to be 

 based on inadequate evidence. The "flowering substance" was said to 

 be extracted from leaves of Xanthium and to induce flower formation 

 when applied as a spray. Extraction of a flower-inducing material from 

 a palm, Washingtonia robusta, inflorescence was also reported (Bonner 

 and Bonner, 1948), but the material could not be obtained a second time. 

 Progress on isolating active compounds is blocked by complete absence 

 of assay methods. Since transfer of the stimuli has so far required con- 

 tact of living tissue, the effective materials might not be very simple and 

 could be protein. 



Harder and Witsch (1940) noted that photoperiodic treatment of cer- 

 tain leaves that were not full-grown when the treatments were started 

 resulted in modifications of their habit. Habits of leaves situated above 

 the treated ones and in the same orthostichy but subjected to long photo- 

 periods were also affected. These morphogenic effects were assumed by 

 Harder and Witsch to result from action of a substance which they called 

 "metaplasin" and which was produced in the leaf under the stimulus of 

 short photoperiods. The pattern of effects produced in leaves above one 

 subjected to short-day treatment led them to conclude that metaplasin 

 was translocated through the vascular system. Harder and Glimmer 

 (1947) obtained supporting evidence for this view by introducing berber- 

 ine sulfate into the vascular tissue of the petiole of a certain leaf and 

 noting that its distribution to structures above could be fairly accurately 

 predicted on the basis of the morphogenic data. The work of Harder 

 and his coworkers illustrates very well the fact that the photoperiodic 

 reaction exerts control over a wide range of both vegetative and repro- 

 ductive processes in the plant. Although they and others have been able 

 to show that certain responses are experimentally separable and thus may 

 result from different immediate control reactions, there seems to be no 

 evidence that these could not stem from the same initial action of light. 



