820 BIOLOGICAL EFFECTS OF RADIATION 



light-stimulated seeds the following showed that the light effect was 

 entirely due to the integrity of the coats: Rumex crispus and Epilobium 

 hirsutum; and the following that the coats played httle or no part in the 

 light-favoring action: Oenothera biennis and Silene densiflora. Bromus 

 squarrosus, Amaranthus retro flexus, Androsace maxima, and Epilobium 

 hirsutum germinated quicker and to a higher percentage in both light and 

 darkness when the coats were broken. For the seeds in which the effect 

 of light was conditioned by the integrity of the coats an increase in partial 

 oxygen pressure above the normal atmosphere increased germination 

 and a decrease in partial oxygen pressure considerably below normal 

 completely inhibited germination. The author concluded that in some 

 seeds and fruits light interfered with oxidation processes within the 

 seeds. In others it favored these processes. The combined action of 

 the light and the coats inhibited the germination in the first case by 

 summation of the hindrance to the oxidation processes. In the second 

 case the favoring effect of light counteracted the inhibiting effect of the 

 coats and increased germination. In seeds and fruits in which the coats 

 did not modify the light sensitiveness, Axentieff assumed that light 

 acted directly on the protoplasm reducing the speed of oxidation in 

 light-hindered seeds and increasing it in light-favored seeds. There is 

 considerable experimental evidence for the coat effects postulated by 

 Axentieff, but direct experimental evidence is lacking for the postulated 

 effect of light upon oxidation within the protoplasm. 



The mechanism by which light affects germination needs much addi- 

 tional study. It is not impossible that the action of light is upon the coats 

 in some cases, upon the living protoplasm in others, and upon both in 

 still others. 



SUMMARY 



A. Light favors the germination of a large number of seeds and 

 fruits. Among these are Viscum album together with many other Loran- 

 thaceae and epiphytes, all Gesneriaceae studied to date, many grasses, 

 various species of Oenothera and Epilobium, Ranunculus sceleratus, Lyth- 

 rum salicaria, and L. hyssopifolia. Viscum album and Arceuthobium 

 oxycedri will not germinate at all without light. The former is killed 

 in darkness within a few weeks, while the latter endures darkness for a 

 longer period. Of 964 species of seeds studied by Kinzel, 672 or about 

 70 per cent were favored by light under the conditions used in his 

 experiments. 



B. Light interferes with the germination of many seeds and fruits. 

 Among these are several species of Phacelia and other Hydrophyllaceae, 

 3 species of Nigella, several species of Allium and most other Liliaceae. 

 Of 964 species of seeds and fruits tested, Kinzel found 258 inhibited by 

 light under the conditions of his experiments. 



