DORMANCY IN SEEDS 77 



the changes brought about in seeds and fruits by agents and conditions 

 which substitute for light." 



The recent findings of FUnt and associates 45. 46, 47. 48 qj^ ^^j^g efifect of 

 different portions of the spectrum on the germination of lettuce seeds are 

 of great interest. These findings deserve special consideration because of 

 the excellence of technique on which they are based. In batches of freshly 

 imbibed lettuce seed that required light for germination, a few seconds' 

 exposure to light induced germination. The sensitiveness approached that 

 of a photographic plate. The region 5200 to 7000 A (red, orange, and 

 yellow) was stimulative, mth the critical wave length at about 6600 A; 

 the region 4200 to 5200 A (green, blue, and violet) was inhibitive, with 

 the critical wave lengths at about 4400 and 4800 A; and the band 7000 to 

 8600 A (mainly infrared) was even more inhibitive, with the critical wave 

 length at about 7600 A. The critical regions for inhibition of germination 

 in the visible spectrum are about the same as those for induction of photo- 

 tropic curvature and for the inhibition of growth of plant organs. The 

 critical inhibitive region in the infrared was not associated with assimila- 



, o 



tion or temperature effects, and 7600 A did not induce phototropic response 

 in lettuce seedlings. Keeping seeds in a dark germinator at 5° C (41° F) 

 for several weeks did not modify then- sensitiveness to light; but keeping 

 them in a dark germinator at 25° C (77° F) for 24 hours did so alter their 

 sensitiveness to light that they would not respond to standard illumina- 

 tion. As we shall see later, a germinator at 5° C (41° F) is an excellent 

 condition for after-ripening many dormant seeds, while a germinator at 

 20° to 25° C (68° to 77° F) maintains many dormant seeds in status quo. 

 It is well established that many fern spores require light for germina- 

 tion. Orth *^ finds two groups of fern spores in respect to their response 

 to light: those that germinate in various bands within the region 550 to 

 710 m/i, and have brown exospores, and those that germinate as above 

 and also in ultraviolet light, and have colorless exospores and much caro- 

 tene in the cells. Within the generally favoring band there are hindering 

 and favoring regions. In the short end of the spectrum various bands of 

 inhibiting rays appear. The same is true in the infrared around 800, 

 1000, and 2400 m/i. Fern spores germinate in light that passes through 

 green leaves, unlike light-sensitive seeds. This is due to the strongly 

 favoring action of green-yellow overcoming the inhibiting action of the 

 infrared. Because of the several bands of favoring or inhibiting action in 

 the spectrum, Orth concludes that the action of light on germination of 

 fern spores cannot be explained on the basis of quanta, as Kommerell ^^ 

 has attempted to do for seeds, but on the basis of the specific effects of 

 various bands of the spectrum on the spores. Flint's results on lettuce 

 seed would seem to justify the same conclusion for seeds. Raleigh ^^ 

 showed that thiourea forced the germination of dormant lettuce seeds 

 (Lactuca saliva) in darkness, and in L. Serriola it increased the germina- 

 tion in both light and darkness. 



