Growth Substances 397 



and Bonner (1938) reported that this effect could be produced in 

 Xanthium through a barrier of lens paper without actual union of tissues. 

 Withrow and Withrow ( 1943), however, failed to confirm this and showed 

 that where transmission of the flowering stimulus had occurred there 

 had been a slight fusion between cells which had grown through the lens 

 paper. Nevertheless, in grafting experiments like these, a substance evi- 

 dently passes from scion to stock across the graft union and induces 

 flowering. To such a substance the name florigen has been applied. 



Other experiments in photoperiodism also suggest the operation of such 

 a flower-forming substance. Cajlachjan (1938), by localizing the recep- 

 tion of the photoperiodic stimulus, showed that this was received by the 

 leaves but was effective in flower induction at growing points considerably 

 distant and had therefore apparently passed, as a specific substance, down 

 the petiole, along the stem, and into a lateral branch. Borthwick, Parker, 

 and Heinze (1941) with soybeans found similar results. Harder (1948) 

 observed that in Kalanchoe this substance passes directly down the stem 

 from the site of induction but does not cross it, so that one side of the 

 plant flowers but the opposite one does not. 



Unlike auxin, the movement of which is usually polar, the flower- 

 inducing agent seems able to travel in any direction in the plant. Since 

 local applications of cold, heat, or narcotics reduce or inhibit the trans- 

 port of such substances from centers of production to those of action, 

 it seems probable that living tissue is involved, a conclusion supported 

 by girdling experiments of Galston ( 1949 ) and others, who showed that 

 the floral stimulus cannot pass across a water gap. 



From the leaf of a unifoliate species of Streptocarpus that was ready to 

 flower, Oehlkers (1955) made a series of cuttings. Those from the base 

 of the leaf produced flowers at once, those from a little farther up pro- 

 duced them soon, and cuttings from near the tip formed only vegetative 

 shoots. Oehlkers believes this was because of the differential distribution 

 of a flower-forming substance. 



Genie differences may also be involved. One variety of Hyoscyamus 

 niger (henbane) is biennial and does not flower until its second year. 

 Another variety is annual. If from the annual form a flowering scion, or 

 leaf from one, is grafted into a plant of the biennial race during its first 

 year, the latter will flower in this season (p. 264). It was also shown that 

 the substance here concerned was not limited in action to this species for 

 a flowering scion of tobacco or petunia (genera in the same family) has 

 the same effect on biennial Hyoscyamus. 



This nonspecificity of the flowering stimulus is also evident in certain 

 host-parasite relationships. Orobanche minor (one of the broomrapes), 

 when parasitizing clover, flowers only when the host plant flowers ( Holds- 

 worth and Nutman, 1947). Cuscuta Gronovii (dodder) flowers only 



