84 • Floral Hormones and the Induced State 



duced state, we find that it is transient in certain plants— that is, 

 they may require almost continuous exposure to the appropriate 

 photoperiod in order to flower— and remarkably long-lived in others 

 (see, for example, Doorenbos and Wellensiek, 1959; Chouard, 1957). 

 Probably most plants are at neither extreme but, like Biloxi soy- 

 bean, revert to vegetative growth after flowering over a period pro- 

 portional to the previous photoperiodic treatment (Borthwick and 

 Parker, 1938a; Hamner, 1940). For obvious reasons, however, the 

 induced state has been studied chiefly in a few plants in which it 

 is relatively permanent, notably in two SDP, Xanthium and 

 Per ilia. 



The induced state in Xanthium is both persistent and trans- 

 missible from plant to plant. The transfer of a florigen from a single 

 leaf on short days through several grafted plants has already been 

 mentioned, but it is possible to separate the final receptor from 

 the short-day donor in time as well. If a plant induced by short 

 days is grafted to a receptor plant in long days, the latter will 

 flower. If the first graft is broken and the first receptor then grafted 

 to another vegetative plant, that plant will also flower on long 

 days, and so on (see Bonner, 1959a). Thus the induced state, by 

 which is meant here the capacity to continue producing florigen, 

 appears to be transferable from plant to plant along with the 

 florigen itself; this might be called "indirect" induction, in con- 

 trast to direct induction by short days. 



If all the actively growing buds of a single-leaved Xanthium 

 plant are removed before and for a few days after a single short-day 

 cycle, the plant remains vegetative. A given leaf can produce the 

 flowering stimulus, but not over a long period of time; the young 

 leaves and buds can apparently be indirectly induced by older 

 leaves, however, and can themselves either store or continue to 

 produce the stimulus in quantity. The experiments indicating this 

 interaction are too complex to describe here (Salisbury, 1955; 

 Lincoln, Raven, and Hamner, 1958), but suggest that in Xanthium 

 the induced state is not permanently localized but depends on the 

 renewed indirect induction of the younger portions of the plant. 



The situation obtaining in Perilla, as reported by both Lona 

 (1959) and Zeevaart (1958), is quite different. A photoperiodically 

 induced leaf continues to produce florigen throughout its life. It 

 can be grafted onto a plant on long days, bringing it to flower, 



