EFFECTS UPON GERMINATION 793 



CONDITIONS MODIFYING THE EFFECT OF LIGHT UPON GERMINATION 



In no other field of plant physiology is there as much disagreement in 

 results as in the effect of light upon germination. For seeds of a given 

 species of plant one author may find light favorable, another may find it 

 indifferent, and a third detrimental. Most of these discrepancies can 

 be explained by the manner in which the internal and external condi- 

 tions mentioned above modify the sensitiveness of seeds to light. The 

 effect of these conditions and the significance of phylogenetic and eco- 

 logical relationships upon Ught sensitiveness of seeds can best be under- 

 stood by considering in detail a number of the seeds that have been 

 most investigated. Let us discuss first several kinds of seeds that are 

 favored in germination by light and then several sorts that are hindered. 



SEEDS THAT ARE FAVORED BY LIGHT 



Loranthaceae and Other Epiphytic Forms. — Since Peyritch and Wiesner 

 (103, pages 182 and 183) discovered that light was necessary for the 

 germination of Viscum album seeds, seeds of several other Loranthaceae 

 have been studied as to their behavior toward light and other factors. 

 Seeds of epiphytes belonging to other families of plants also have received 

 some attention. 



Wiesner (104) showed that the germination of Viscum album seeds 

 increased with light intensity. His experiment extended from March 



24 to April 22. During this period the intensity of the daylight varied 

 from 0.016 to 0.375 of a Wiesner chemical light unit. With an intensity 

 of 0.142 of a unit seeds gave 42 per cent germination; with 0.024 of a unit, 



25 per cent; with 0.015 of a unit, 5 per cent; and with 0.0013 of a unit, 

 per cent. The minimum intensity necessary for the germination of 

 Viscum album seeds was about 0.04 of the maximum intensity of sunlight 

 at Vienna during the germination period. The germination percentage 

 increased as the light intensity rose, to approximately one-half the 

 maximum intensity of sunlight during that period. In continuous 

 darkness the seeds died within a few weeks. The minimum intensity 

 necessary for growth of the hypocotyl was a little lower than for germina- 

 tion, and the rate of growth of this organ also increased with increased 

 light intensity. The seeds germinated in the mucilaginous fruits if the 

 light intensity was adequate. 



Wiesner (105) made a fuller study of the germination of seeds of two 

 European mistletoes, Viscum album and Loranthus curopaeus, and of 

 four tropical species, Viscum articulatum, V. orientale, Loranthus repandus, 

 and L. pentandrus. Seeds of the two European species were surrounded 

 by fruits rich in mucilage, required light for germination, would not 

 germinate in the presence of liquid water and had 6 months' rest period 

 in nature. Seeds of the tropical species were surrounded by fruits bearing 

 little mucilage, germinated in darkness although somewhat favored by 



