76 GROWTH OF PLANTS 



favor the germination of the following light-favored fruits and seeds in 

 darkness: Poa, Ranunculus, Epilobium, Lythrum, and the Gesneriaceae. The 

 light-inhibited seeds of Phacelia and Nigella are not favored by nitrates. 



" (e) Weak acids substitute for light in part in the light-favored seeds 

 of Lythrum, Scrophularia, Verbascum, and Epilobium. 



" (/) Either daily intermittent or high constant temperatures substitute 

 for light in various light-favored seeds. The most favorable intermittent 

 temperatures give better germination of Poa achenes than light with any 

 constant temperatures. Light and nitrates increase the germination of 

 Poa compressa achenes somewhat at the most favorable intermittent tem- 

 peratures. Intermittent temperatures replace light with after-ripened 

 Chloris achenes with hulls intact, but not with non-after-ripened or ' dunkel- 

 hart' achenes. With seeds of Epilobium, Oenothera, and others intermittent 

 temperatures substitute fully for light. 



"(£■) When light-favored achenes of Chloris are kept for a time in a 

 dark germinator, they are changed in a manner that makes them inca- 

 pable of germination later even in hght. Such seeds are said to be ' dunkel- 

 hart.' 'Dunkelhart' achenes can be forced to germinate by breaking the 

 coats, increasing oxygen pressures, and other treatments. When light- 

 inhibited seeds of Nigella are kept for a time in a light germinator at a 

 temperature above 20° C, they are changed in such a manner that makes 

 them incapable of germination later even in darkness. Elinzel spoke of 

 such seeds as 'lichthart.' 'Lichthart' seeds can be forced to germinate by 

 breaking the coats, or still better by other treatments. Imbibed Phacelia 

 seeds also become 'lichthart' when exposed to light. 



" {F) If imbibed Ranunculus sceleratus seeds are exposed to light, dried, 

 and later placed in a dark germinator \vith intermittent temperatures, 

 they still show the favorable effect of the light exposure. Chloris achenes 

 also show this latent light effect. Since the light exposure of the seeds 

 during ripening in the capsules varies mth the weather, the rate of dry- 

 ing of seeds in the capsules, and the position of the capsule on the plant, 

 Wieser concluded that the latent light effect may account in part for the 

 great variation in the amount of light required for the germination of 

 different collections of the same species of light-favored seeds. 



" (G) Several theories have been offered to explain the favoring or in- 

 hibiting action of light upon the germination of seeds and fruits. Most of 

 these theories postulate that the action of the light is upon the living 

 endosperm or embryo, but some of them assert that the action is upon the 

 non-living coats. None of these theories has adequate evidence for even a 

 single species of seeds. It is not improbable that light has its effective 

 action upon the endosperm and embryo of some seeds, upon the coats of 

 others, and upon both in still others. There is need of a very thorough 

 and detailed chemical, microchemical, and physiological study of the effect 

 of light upon the coats and living portions of several light-favored and 

 light-inhibited seeds and fruits. There is also need of a similar study of 



