SEED DORMANCY AND DORMANCY BREAKING 181 



present in 'white' light may be responsible for this, since it has 

 been found that short and prolonged Far Red irradiations differ 

 in their effect. In the Amaranthus seeds, a short illumination 

 with Far Red is reversible by a subsquent red irradiation, 

 whereas the inhibition by Far Red given over a prolonged period 

 is not reversed by an immediately following illumination with 

 red light^^. Therefore, when white light is given over a prolonged 

 period, the 'prolonged Far Red' effect may be dominant over the 

 stimulatory red effect and cause the inhibition. 



The effect of the prolonged Far Red irradiation, however, 

 decreases with time : as mentioned, red light given immediately 

 after the prolonged Far Red will be without effect: but when the 

 seeds are kept for some time in darkness after the Far Red 

 illumination and only then exposed to red-light, germination 

 will occur. 



A somewhat similar difference between short and prolonged 

 Far Red irradiation has been found also in certain lettuce seeds. 

 The seeds of the 'Progress' variety are light sensitive only 

 when very young. Older seeds have no light requirement and 

 when imbibed, will germinate readily both in the dark and in the 

 light. Red light has no effect on those seeds, because of the 

 already high 'dark germination', but also a short Far Red 

 illumination is without effect and will not cause inhibition of 

 germination. However, when the seeds are exposed to prolonged 

 Far Red irradiation, their germination is almost completely 

 inhibited. This inhibition is reversible by red in-adiation but 

 much stronger doses of red light are required than in the case of 

 the usual 'Red-Far Red reaction' (Table I). Both the facts, that 

 in this case (1) a short Far Red irradiation is without effect, 

 whereas prolonged Far Red irradiation inhibits, and (2) that 

 Red in much stronger doses than usual is needed to overcome 

 the Far Red effect, may be taken as an additional indication 

 that the 'prolonged Far Red' effect is different from the effect 

 of Far Red in the well known short Red-Far Red reactionio. 

 Still, it has yet to be shown definitely whether this difference 

 exists in reality or whether the mechanism of the short Red-Far 



References p. 190 



