70 • Floral Hormones and the Induced State 



there was such union, and all membranes that would prevent 

 actual "taking" of the graft also prevented transmission. 



Chailakhyan (1937) had already concluded from experiments 

 with Perilla and Chrysanthemum, and the Withrows (1943) con- 

 firmed with Xanthium, that florigen movement occurred only 

 through the "bark"— the phloem and cortical tissue. If this was 

 removed in ringing or girdling experiments, no movement of the 

 floral stimulus across the girdle was observed, although water con- 

 tinued to pass through the xylem (wood) and the shoots remained 

 healthy. Presumably the major route of transport is the phloem 

 itself, in which most organic substances are transported; but we 

 will return to this question later. 



Questions obvious from the start of this kind of research are 

 whether the florigen of one kind of plant is effective on another 

 and, more particularly, whether that of an SDP will act on an LDP 

 and vice versa. Auxins are not species-specific, but such questions 

 are more difficult to answer with respect to flowering hormones, 

 transmissible from plant to plant only by grafting. Successful grafts 

 are generally possible only between closely related plants so that no 

 completely general answer can be given. Within these limitations, 

 however, the floral stimulus produced by one species is often 

 effective on other, closely related species. 



Maryland Mammoth tobacco and annual Hyoscyamus niger 

 are members of the same family (Solanaceae) and can be success- 

 fully grafted. In such a graft partnership, the LDP Hyoscyamus 

 will flower under short-day conditions if the SDP tobacco is also 

 kept under short days, but not if the tobacco is exposed to long 

 days. That is, under short days the tobacco is itself induced and 

 serves as the donor of stimulus of florigen to Hyoscyamus. Con- 

 versely, the tobacco can be made to flower under long-day condi- 

 tions if the Hyoscyamus is induced by also being kept under long 

 days, but not if the Hyoscyamus receives short days. Here Hyoscy- 

 amus becomes the donor and Maryland Mammoth the receptor 

 (Lang and Melchers, 1947; see Lang, 1952). The simplest conclu- 

 sion is of course that the florigens produced by Hyoscyamus in 

 long days and by Maryland Mammoth tobacco in short days are 

 physiologically equivalent if not identical. 



There are many similar experiments in the literature. The 

 SDP Xanthium, lor example, can be made to (lower on long days 



