794 BIOLOGICAL EFFECTS OF RADIATION 



light, needed liquid water for germination and had no rest period. They 

 germinated in 2 to 5 days, except for L. pentandrus which required a few 

 weeks for germination because of slow mobilization of food reserves. 

 The germination characters of these seeds showed natural adaptations to 

 their habitats. Their behavior seemed to be determined by ecological 

 conditions rather than by phylogenetic relations. 



Wiesner thought that the 6 months' rest in nature necessary for 

 the germination of Viscum album seeds was due to slow mobilization of 

 reserves, need of light of considerable intensity, and presence of inhibiting 

 substances in the fruits. In 1897 he found that the rest period of some 

 of the seeds could be greatly shortened, even to 1 month, by using 

 optimum conditions: temperatures of 15° to 20°C., good light, and dry 

 air. Immature seeds germinated more quickly than ripe ones. 



Contrary to his earlier claims, Wiesner (106) found that Loranthus 

 europaeus seeds germinated in darkness, although they were favored in 

 speed and percentage germination by light. This result was later 

 confirmed by Kinzel (51, Suppl. II, page 67) and Mayr (72). Loranthus 

 europaeus seeds had a shorter rest period than Viscum album. The imma- 

 ture seeds of this species also germinated more promptly than mature 

 ones. 



With a few exceptions Heinricher (36, 37, 39, 40) confirmed Wiesner's 

 conclusions on germination conditions for Viscum album seeds. His 

 exceptions were the following: Germination in the open was found to 

 occur during a warm period in February rather than being delayed 

 until April. Germination in the greenhouse was 100 per cent by February 

 12 in contrast to 10 per cent reported by Wiesner. This result Heinricher 

 attributed to the excellent light in his new greenhouses. He germinated 

 Viscum album seeds in contact with liquid water at high temperatures 

 by maintaining the seeds in sterile condition. He claimed that the 

 fruit mucilage hindered germination because it limited the oxygen and 

 water supply to the embryo, rather than because of an inhibiting chemical 

 within the mucilage. He found the minimum temperature for germina- 

 tion to be about 3.8''C. rather than the 8° to 10°C. reported by Wiesner. 

 Heinricher also added some interesting facts not found by Wiesner. The 

 less refrangible half of the visible spectrum favored germination, but the 

 more refrangible half was not effective. A substratum of nutrient 

 gelatin did not favor germination in light or cause germination in dark- 

 ness. Kinzel (51, pages 16 and 17) found nutrient gelatin ineffective for 

 Viscum album and V. minimum seeds. This point is of great interest 

 because, as we shall see later, nutrient solutions often substitute for 

 light in light-favored seeds. Kinzel also germinated V. minimum in 

 darkness under favorable conditions. 



Tubeuf (97) found that germination of the red-berried mistletoe, 

 Viscum cruciatum, was greatly favored by light and was slow and incom- 



