798 BIOLOGICAL EFFECTS OF RADIATION 



coat over the embryo. In this respect " dunkelhart " achenes of Chloris 

 acted Hke "lichthart" seeds of Nigella saliva (46, page 269). 



Gassner (27) found that if imbibed light-favored achenes were exposed 

 to an effective dose of light, dried for a long period, and then placed in a 

 dark germinator, the favoring effect of the light still persisted. This 

 latent light effect was easier to demonstrate with Ranunculus sceleratus 

 seeds for, while they are modified by light at 28°C. constant, they will 

 germinate in light only at intermittent temperatures. It was demon- 

 strable, nevertheless, with Chloris achenes. Gassner believed this 

 latent light effect proved that light did not act as a releasal stimulus 

 but brought about some permanent biochemical change which lasted 

 during a long period of dry storage. 



As noted above, after-ripened achenes with hulls removed germinated 

 equally well in light and dark at high temperatures, but if the hulls were 

 intact, they were greatly favored by light. Achenes with hulls intact 

 germinated readily in darkness in a full atmosphere of oxygen; conse- 

 quently, Gassner (23) concluded that the hulls lowered the oxygen supply 

 to the embryo and thereby prevented germination. The hulls changed 

 the achenes to light requirers by restricting the oxygen supply. Gassner 

 changed after-ripened achenes with the hulls removed to light requirers by 

 wrapping them tightly in wet filter paper. He concluded that the wet 

 filter paper reduced the supply of oxygen to the achenes. Removing the 

 hulls aided germination at both constant and alternating temperatures. 



Daily intermittent temperatures (22) were effective in forcing the 

 germination of after-ripened Chloris achenes with hulls intact but had no 

 effect on such achenes with the hulls removed, for the removal of the 

 hulls alone gave good germination. Intermittent temperatures were not 

 effective with non-after-ripened achenes or "dunkelhart" achenes, 

 but their effectiveness rose with the degree of after-ripening of the achenes 

 that were not "dunkelhart." The best daily intermittent temperatures 

 had a large difference between the low and high temperatures with the 

 low-temperature period long in comparison with the high period. The 

 intermittent temperature of 12°C. for 19 hr. daily and 28°C. for 5 hr. 

 was much more effective in forcing germination in darkness than 12°C. 

 for 5 hr. and 28°C. for 19 hr. Gassner believed that daily intermittent 

 temperatures increased the oxygen supply to the embryo, favoring 

 germination by the same means as removal of the hulls. At the low 

 temperature oxygen accumulated in the achene because of its higher 

 solubility at this temperature and because of low respiration. The 

 high temperature with the high oxygen supply favored quick germination. 

 Gassner (23, 25, 26) found that Knop's solution displaced the need of 

 light for the germination of Chloris achenes. It was effective at all 

 temperatures that permitted germination, while Ught was favorable only 

 at temperatures above 22*0. Table 1 shows the favoring action of 



