814 BIOLOGICAL EFFECTS OF RADIATION 



limited the oxygen supply to the embryo. This Umitation could be 

 partially or entirely removed by increased oxygen pressure or removal 

 of the seed coats. Axentieff (3) pointed out that light interfered with 

 oxidation processes in some plants and stimulated it in others; he sug- 

 gested that Pkacelia and other light-inhibited seeds might be prevented 

 from germination by the joint action of the coats, which restrict the oxy- 

 gen supply to the seed contents, and light, which interferes with oxidation 

 processes within the seeds. 



Lehmann (62) believed that light inhibition generally characterized 

 seeds of the Hydrophyllaceae since Nemophila insignis and its relatives 

 Phacelia tanacetifolia, P. whitlavia, and P. campanularia all were hindered 

 by light. On the other hand, Kuhn (56) showed that the seeds of 

 Hydrolea spinosa were favored by light. No strict relationship usually 

 exists between systematic grouping and light-sensitiveness. 



Nigella. — Kinzel (46) show^ed that at 20°C. seeds of Nigella sativa 

 were hindered in germination by light, and that they became "lichthart" 

 with only 24 hr. exposure. "Lichthart" seeds were incapable of later 

 germination without special treatment. Although light lowered the 

 germination at 10° to 15°C., it did not make the seeds "hchthart." 

 Seeds which had become "Hchthart" could be forced to germinate in 

 part by pricking the coats or by using daily intermittent temperatures 

 of 20° to 30°C. The best way to force "hchthart" seeds was to dry them 

 over CaCU for 24 hr. at 30°C., soak in 1 per cent asparagin and 0.1 per 

 cent papayotin solution for 5 days, prick with a pin, and then after 24 hr. 

 swelling put to germinate at alternating temperatures of 20° to 30°C. 

 Seeds which had been in a light germinator 7 months germinated within 

 14 days after this treatment. Nigella damascena was more sensitive to 

 light than Nigella sativa. According to Kinzel (48) all portions of the 

 visible spectrum inhibited germination of Nigella except green and green- 

 blue, which gave about the same germination as darkness. The habitat 

 in which the seed developed and the stage of after-ripening (51, pages 

 21 and 22) modified both the percentage germination and the degree of 

 sensitiveness to light. 



Lehmann (60) agreed with Kinzel that light rendered Nigella seeds 

 "lichthart," but he claimed that an exposure of 3 days or more was 

 necessary for any marked effect, and that much longer exposures were 

 required for greatest effectiveness. Niethammer (76) found N. damas- 

 cena seeds light-avoiding at all temperatures, old seeds being less so than 

 fresh ones. According to Axentieff (3) the inhibiting action of light on 

 N. arvensis seeds was not entirely dependent upon the integrity of the 

 coats. Bohmer (8) found that under light of 20 M. K. intensity germina- 

 tion of A^. sativa seeds increased somewhat as the oxygen pressure rose, 

 and that 100 per cent oxygen was very favorable for forcing germination 



