800 BIOLOGICAL EFFECTS OF RADIATION 



Because of the latent effect of light on Chloris achenes and Ranunculus 

 seeds, and because of the failure of the Fechner law to apply, Gassner 

 believed that light did not act as a releasal process or stimulus in the sense 

 of Pfeffer or Jost. He also considered untenable Lehmann and Otten- 

 walder's conception that light had a direct or indirect catalytic effect on 

 the seed, thereby increasing the mobilization of reserve materials. The 

 catalytic conception would not explain the effectiveness of nitrates with- 

 out their entering the achenes, or the production of a "dunkelhart" 

 condition in the dark germinator. Gassner thought that unfavorable 

 conditions in the germinator brought about a change in the achene coats 

 which prevented later germination even under favorable conditions. He 

 spoke of this as the "Hemmungsprinzip." Any condition causing quick 

 germination did not give the change in the coats time to occur. Among 

 these conditions were high light intensity along with high temperature, 

 absence of the hulls, high partial oxygen pressure, nitrogen compounds in 

 the substratum, and suitable intermittent temperatures. 



In his work Gassner did not explain how light overcame the limited 

 oxygen supply caused by the hulls. Aside from a change in color he did 

 not define any change in the coats produced during a period in the dark 

 germinator. He failed to determine whether the embryo was modified 

 by the period in the dark germinator so that it became incapable of later 

 overcoming the resistance of the coat to germination. Changes occurring 

 in both the coats and embryos during the period in the dark germinators 

 were possible to investigate and should have been investigated. Davis 

 (15) showed that embryos of Xanthium seeds with coats intact could be 

 thrown into a partially dormant state by certain unfavorable conditions 

 in the germinator. The partially dormant embryos would not grow when 

 the coats were intact even under favorable conditions, and grew very 

 tardily when the coats were removed. Davis (14) obtained similar but 

 more striking effects with naturally dormant but after-ripened embryos 

 of Ambrosia when they were placed in a germinator with the coats intact 

 at 28°C. or above. In both of these seeds high oxygen pressure forced 

 germination if the embryos were not dormant, and the seed coats were 

 intact. 



Gassner (21, pages 364, 504 to 512) studied the effect of light and other 

 conditions upon the germination of the achenes of three other grasses 

 of the pampas. The germination of Chloris distichophylla was favored 

 by light, and the germination was improved in both light and dark by 

 after-ripening in dry storage. The optimum temperature for germination 

 was between 35° and 40°C., the minimum about 25°C., and the maximum 

 45°C. Chloris distichophylla achenes were not so sensitive to a dark 

 germinator as those of Chloris ciliata. Stenotaphrum glahrum achenes 

 were favored only sUghtly by light. Like all the pampas grasses men- 

 tioned in this section, they are attuned to high temperatures: minimum 



