804 



BIOLOGICAL EFFECTS OF RADIATION 



achenes. His results are reported in Table 5. Samples I and III 

 were commercial samples from America. Samples II and IV were 

 collected at Munich ; II ripened in sunny weather, and IV in rain. Figures 

 give germinations in one month, except those in parentheses which are 

 for two months. All temperatures were intermittent (20°C. 18 hr. 

 daily and 30°C. 6 hr.) except the one designated, which was constant 

 at 20°C. 



Table 5 



Light condition 



Darkness . . 



Light 



Dark red. . 

 Orange .... 

 Yellow.... 



Green 



Bright bkie 

 Dark blue . . 

 Dark violet 



I 



36 



49 (66) 

 21 (70) 

 52 (65) 

 12 (63) 

 62 (77) 

 34 (68) 

 (62) 

 (27) 



I (20°) 







47 (70) 

 24 (70) 

 65 (70) 

 13 (61) 

 75 (79) 

 32 (73) 

 (65) 

 (30) 



II 



96 

 91 

 90 

 89 

 92 

 90 

 89 

 88 

 89 



III 



85 

 97 

 86 

 95 

 97 

 90 

 57 

 27 

 43 



IV 



92 

 90 



86 

 93 

 91 



87 

 83 

 88 

 83 



Darkness gave better germination in samples II and IV than the full 

 visible spectrum and somewhat poorer results in the other two samples. 

 Light was more effective than the red end of the spectrum. Blue and 

 violet rays were very unfavorable in samples I and III even with alter- 

 nating temperatures, but in samples II and IV the short end of the 

 spectrum showed little decrease in germination. This work again showed 

 the significance of intermittent temperatures and the extent to which 

 they dominate light effects. 



Several attempts have been made to explain the mechanism by which 

 light favors germination of Poa achenes, but no explanation except the 

 first mentioned below has been supported by enough facts to justify 

 its serious consideration. Much of the effect of direct sunlight doubt- 

 less is due to its producing favorable intermittent temperatures, as several 

 authors (31, 32, 46, 84) believed. Diffuse Ught and artificial light of 

 low intensity increased germination somewhat. It is not likely that they 

 had their effect by producing intermittent temperatures. Reiling (87) 

 suggested that light transformed and activated the reserve materials of 

 the endosperm, but he did not get convincing evidence. Gassner (28) 

 thought that in Poa light prevented the development of the "Hem- 

 mungsvorgiinge," as he believed it did in Chloris achenes. Kummer (58) 

 claimed that grass achenes with fats having a low acid number needed 

 light, while achenes with fats having a high acid number germinated in 

 darkness. This iield even for seeds of the same species. The various 

 hypotheses offered above for explaining the action of light are worthy 

 of investigation but are by no means established. 



