792 BIOLOGICAL EFFECTS OF RADIATION 



Nobbe (80) vigorously contradicted Stebler's results. He reported 

 that seeds of Poa pratensis, Phleum pratense, and Zea mays germinated 

 better in darkness than in light and that seeds of Dactylis glomerata 

 germinated equally well under both conditions. He drew the sweepmg 

 conclusion that the method of seed testing in darkness found in nature 

 was reliable even for grasses and preferable to testing in light, since 

 under identical conditions the process in darkness was quicker, more 

 certain, and more regular, and constant temperature and moisture 

 conditions were easier to maintain. 



Nobbe's opinion did not long dominate the field, for Liebenberg (69), 

 Jonsson (45), Weinzierl (99), Laschke (59), Hiltner (42), and Pickholz 

 (84) extended and confirmed Stebler's results in their studies of the 

 effects of after-ripening and constant and intermittent temperatures on 

 light-sensitiveness of grass seeds. Later investigations have shown that 

 the germination of seeds of many plant families, gymnosperms as well 

 as angiosperms, is favored by light. A large number of internal and 

 external conditions modify the sensitiveness of seeds to light, or even 

 annul or reverse the light effects. Among these factors are maturity of 

 the seeds, stage of after-ripening, condition and integrity of the seed 

 coats, region where the seed grew, partial oxygen pressure, 

 temperature of the germination bed, whether constant or fluctuating, 

 acidity of the substratum, and the presence of nitrates or other nitrogen 

 compounds. Lehmann and Aichele (66, pages 432 to 461) give an excel- 

 lent critical summary of the early literature on the effect of light on seed 

 germination. 



While for the germination of many sorts of seeds light is required or 

 favorable, it is detrimental to the germination of some other kinds. 

 Heinricher (34) found that darkness increased the germination velocity 

 and percentage of seeds of Acanthostachys strohilacea. Light proved 

 especially detrimental to germination capacity. Remer (88) showed 

 that diffuse light reduced to a marked degree the germination of Phacelia 

 tanacetifolia seeds. In full sunlight under similar temperature and 

 moisture conditions seeds covered with 0.5 to 1 cm. of soil gave 97 per 

 cent germination, and seeds on top of the soil gave 15 per cent germina- 

 tion. Following Remer's work many investigators studied the germina- 

 tion of Phacelia seeds in light and darkness under many conditions. 

 Seeds of Phacelia tanacetifolia became the classical example of light- 

 inhibited seeds. 



Kinzel (46), in 1907, found that Nigella sativa needed darkness 

 for good germination. Later investigations by Kinzel and others showed 

 that the germination of various species of Dianthus and Allium, Lychnis 

 lapponica, Soldanella alpina, Primula spectabilis, and many others was 

 hindered by light. 



