The Biochemistry of Induction • 89 



light-intensity) processes in most plants studied appear at least to 

 have the red, far-red reversible system in common, but its bio- 

 chemical function is unknown. Again, the role of endogenous 

 rhythms is uncertain. 



Many specific mechanisms have been proposed for various 

 processes in induction, mostly involving transformations and inter- 

 actions of hypothetical substances. As Lang (1952) has pointed out, 

 they are often little more than generalized restatements of particu- 

 lar data. Since expositions of these hypotheses abound in the 

 reviews and papers cited, no attempt will be made to represent 

 them here. Instead we will briefly consider some of the general areas 

 of investigation involved. 



One of the earliest and still most favored ideas is that auxin 

 plays a major part in photoperiodic induction and flower initiation. 

 The possibility that induction might be caused by a change in 

 auxin content was tested by Chailakhyan and Zhdanova (1938); 

 they concluded that this was unlikely since auxin content in a 

 number of plants was greater on long than on short days, irre- 

 spective of whether they were LDP or SDP. More recent work of 

 the kind has confirmed their general conclusions (see Hillman and 

 Galston, 1961; Doorenbos and Wellensiek, 1959), but a major prob- 

 lem is the multiplicity of auxins as well as other growth-promoting 

 and growth-inhibiting substances in plants; it is difficult to be sure 

 that all the relevant compounds have been assayed in a given 

 investigation. Thus changes in one or another identified or un- 

 identified substance may or may not be correlated either with a 

 change from one photoperiod to another or with flowering response, 

 but are not easily interpretable as the cause of flowering (Cooke, 

 1954; Vlitos and Meudt, 1954). 



A study by Harada and Nitsch (1959a), in which paper 

 chromatography was used to separate and help identify various 

 compounds, illustrates the complexity of the situation. They fol- 

 lowed changes in the amounts of growth substances extractable 

 from an LDP, an SDP, and a vernalizable plant at various times 

 during or after induction. In each plant there was a number (3 to 

 6, perhaps more) of active substances; the levels of some changed 

 in such a way as to suggest that they might be the cause of the 

 developmental changes rather than being merely correlated with 

 them. These results are only suggestive at present, but intensive 



