72 • Floral Hormones and the Induced State 



same family (Convolvulaceae— morning glories) and then inducing 

 the latter (Lam et al., 1959). 



The occurrence of transmissible flowering stimuli is not con- 

 fined to photoperiodic plants. This is of course implicit in the 

 fact, noted earlier, that many plants are only quantitatively photo- 

 periodic, or are photoperiodic only under certain conditions, 

 whereas some are completely daylength-indifferent; the processes 

 leading to flowering may or may not be under photoperiodic 

 control and still have the same end result. Lang (1952) has reviewed 

 work in which daylength-indifferent plants can serve as donors of 

 a flowering stimulus to closely related LDP or SDP. 



Not all results on the transmission of flowering stimuli have 

 been straightforward, and before proceeding further it is well to 

 keep the fundamental difficulty in mind. Auxins can be obtained 

 from plants either by diffusion from cut tissues, as previously 

 described, or by extraction. They can then be reapplied and will 

 cause growth in responsive tissue. This makes possible not only the 

 identification and quantitative assay of naturally occurring auxins 

 but also the study of the biochemistry of their origin and function. 

 Not so for the hypothetical florigen— which remains hypothetical 

 for the very reason that, with one possible exception, no work to 

 date has successfully isolated it from the living plant; attempts to 

 do so will be discussed in the following chapter. Thus it has not 

 been possible to study flowering hormones chemically, and all the 

 evidence is necessarily circumstantial. Hence the use in this chapter 

 of all sorts of terms-florigen, floral hormones, flowering stimuli, and 

 so on— to avoid implying a precision that does not exist. We must 

 now pay closer attention to the experimental systems involved in 

 such work— the plants themselves— following which we can return 

 more critically to the question of whether floral hormones actually 

 exist. 



TRANSLOCATION OF FLOWERING HORMONES 



The conclusion that florigen moves only through living tissues 

 is based on observations besides those already presented. Borthwick, 

 Parker, and Heinze (1941) showed that a soybean plant defoliated 

 to only a single leaf could flower in short days, but not if the petiole 

 was chilled to 3 C C. This was true even if another leaf was left on 



