180 S. KLEIN 



dormancy can be broken by changes in gas pressure or by 

 removal of the endosperm points to this structure as a factor in 

 dormancy. I shall return to the influence of the seed coats on 

 germination of the lettuce seed later. 



Part, if not all, of the light mechanism is connected with the 

 well known reversible Red-Far Red reaction. The light effective 

 in bringing about germination is red light of about 6500-6800 A, 

 the promoting effect of which can be completely overcome by 

 illumination by Far Red light, of wave length 7200-7500 A, and 

 vice versa^' '^' i^. It is the same reaction that is active not only 

 in the control of germination of a large number of seeds, but 

 also in a wide array of other phenomena and can be regarded 

 'as a general factor of growth control'", thereby relating seed 

 germination to other biological phenomena. 



In this reaction, a single relatively short illumination is 

 necessary to bring about the promoting or inhibitory effect. The 

 sensitivity to this illumination depends on a number of factors, 

 among others the time lapse between imbibition in darkness and 

 illumination. At a temperature of 26°, lettuce seeds start to 

 respond to light as little as approximately 10 min after imbibi- 

 tion and the sensitivity rises for approximately 8 h, and then 

 decreases^^. This time curve is not typical for all light-sensitive 

 seeds. For Amarauthus seeds it was shown that several days are 

 needed before sensitivity reaches its peak, but in this case the 

 sensitivity is maintained even after two months^"^. 



As regards illumination with white light, a distinction should 

 probably be made between the effects of a single short illumi- 

 nation and those of prolonged continuous illuminationi^. For 

 example, Kadman-Zahavi^-^ found in Amarauthus retroflexus, at 

 a certain light intensity, 32% germination in the dark, 3% in 

 continuous light and 92% after a single short illumination. In 

 general, the main difference seems to be that in the case of short 

 illuminations with white light, germination rises with light 

 intensity, whereas with continuous illumination, germination 

 response is inversely proportional to it^^. i5. Attempts have been 

 made to explain this, assuming that the Far Red radiation 



