398 Morpho genetic Factors 



in a long day if it is parasitic on the long-day plant Calendula, but in a 

 short day if it is parasitic on the short-day plant Cosmos (von Denffer, 



1948). 



These various lines of evidence, about which a great body of facts has 

 now been gathered, suggest that there are one or more specifically flower- 

 forming substances. None of these florigens has yet been isolated nor is 

 there any knowledge as to their chemical nature. Some substance that 

 under certain conditions stimulates flowering is certainly able to pass 

 across a graft union and thus seems hormonal in character. This sub- 

 stance is evidently closely involved with photoperiodism, though what it 

 does depends to a great extent on the amount of auxin or other growth 

 substances present. Thus J. Bonner and Thurlow (1949) completely pre- 

 vented flowering in the short-day plant Xanthium canadense, grown 

 under short days, by spraying it with auxin, and leaves thus treated did 

 not transmit the flowering stimulus by grafting. Substances that antago- 

 nize auxin action, such as triiodobenzoic acid, increased flowering in soy- 

 beans (Galston, 1947). De Zeeuw and Leopold (1956) report that low 

 auxin concentrations applied to two short-day species promoted floral 

 initiation if applied before the induction period but are less effective 

 afterward. In a Xanthium plant defoliated to a single leaf, Salisbury 

 ( 1955 ) found that auxin inhibited flowering if applied before the flower- 

 ing stimulus (produced by photoperiodic induction) had been com- 

 pletely translocated from the leaf but promoted it if applied afterward. 

 Leopold and Guernsey ( 19535 ) , using the position of the first flower in 

 peas as a measure of flower initiation, observed that a number of sub- 

 stances, notably sucrose, malic acid, and arginine, tended to inhibit flower- 

 ing but that this inhibition was removed by auxin. Although flowering 

 is usually an "all or none" reaction, structures intermediate between 

 flowers and vegetative shoots sometimes occur. Such phyllody has been 

 produced through manipulation of the flowering stimulus by Harder and 

 his colleagues (1947). Gibberellin is often effective in flower induction 

 (Lang, 1957). 



The pineapple (Clark and Kerns, 1942) produces flowers abundantly 

 if naphthaleneacetic acid or certain other growth substances are applied 

 by spray to the center of the plant. Van Overbeek (1946«) has shown 

 that plants thus treated will flower under long days, which ordinarily 

 inhibit flowering in pineapple. In the sweet potato, also, Howell and 

 Wittwer ( 1955 ) reported that flowering can be induced experimentally by 

 a growth substance. 



The problem of the relation of growth substances to flowering is evi- 

 dently a complex one. It is the basis of a considerable literature, much 

 of which can be found in Melchers and Lang (1948), Lang (1952), Bon- 

 ner and Liverman ( 1953), and Liverman ( 1955). 



Something analogous to the control of flowering by specific substances 



