Photoperiodism and Light Quality • 39 



the theory (Nakayama, 1958). However, all this is true only when 

 the cotyledons are the light-responsive organs. Older plants, in 

 which the true leaves perceive the light, respond in the same way 

 as Xanthium (Nakayama, Borthwick, and Hendricks, 1960). These 

 observations provide an opportunity for studying the precise ways 

 in which the red, far-red system may be linked to flowering, if the 

 operative differences between the cotyledons and the true leaves 

 can be discovered. 



A point requiring further comment is that white light acts 

 more or less like red. This is not surprising for fluorescent light 

 sources since their far-red emission is very low, but both incandes- 

 cent light and sunlight have a high proportion of far-red. Their 

 action as red light is probably due in part to the proportion of red 

 to far-red, in part to the relative sensitivities of the two forms, and 

 also to the fact, mentioned previously, that prolonged exposures to 

 far-red may have an action more like red than short exposures. The 

 latter has been explained (see Borthwick, 1959) as being due to the 

 maintenance of a small amount of the far-red-absorbing form in 

 equilibrium with the red-absorbing form during far-red radiation, 

 since the absorption spectra of the two forms must overlap. Thus 

 darkness following the far-red treatment is needed to allow the 

 conversion to the red-absorbing form to be completed by the 

 thermal process. It is, however, not strictly true that all white light 

 sources are equivalent for photoperiodism. Fluorescent and in- 

 candescent light differ considerably in their effects on both flower- 

 ing and vegetative growth when used to lengthen light periods, and 

 the differences can be ascribed to the different far-red emissions 

 of the two sources (Downs, 1959; Downs et al., 1959). 



NATURE AND FUNCTION OF THE 

 RED, FAR-RED PIGMENT 



Many effects of low-intensity red light on plants are now known 

 to be reversible by far-red, but a discussion of the red, far-red 

 control of vegetative growth— so-called photomorphogenesis— would 

 occupy too much space here. References to the abundant literature 

 on it are to be found in most reviews on photoperiodism; a 

 particularly good introduction is Withrow's own article in Withrow 

 (1959). Much speculation and calculation has in the past been 



