822 BIOLOGICAL EFFECTS OF RADIATION 



E. When light-favored achenes of Chloris are kept for a time in a dark 

 germinator, they are changed in a manner that makes them incapable of 

 germination later even in light. Such seeds are said to be "dunkelhart." 

 "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 incapa- 

 ble of germination later even in darkness. Kinzel 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 with intermittent temperatures, 

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

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

 during ripening in the capsules varies with the weather, the rate of drying 

 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 

 inhibiting 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 nonliving 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 the changes brought about in seeds and fruits by agents 

 and conditions which substitute for light. 



REFERENCES 



1. Andersen, Alice M. The effect of removing the glumes on the germination of 

 seeds of Poa compressa. Amer. Jour. Bot. 19 : 835-836. 1932. 



2. Axentieff, B. N. t)ber die Wirkung von Samenausziigen auf das Keimen von 

 Samen. Jour. Soc. Bot. Russie 12 : 291-306. 1928. 



3. Axentieff, B. N. tJber die Rolle der Schalen von Samen und Friichten, die 

 bei der Kcimung auf Licht reagieren. Beih. Bot. Centralbl. Abt. 1. 46 : 1 19-202. 

 1929. 



4. Axentieff, B. N. Uber die Wirkung von Samenextrakt auf das Keimen der 

 Samen. Proc. Internat. Se?d Test. Assoc. 2 (11-12): 149. 1930. 



5. Baar, Henryk. Uber den Einfluss des Lichtes auf die Samenkeimung und seine 

 Abhangigkeit von andern Faktoren. Sitzungsb. Akad. Wiss. Wien. Math. 

 Naturwiss. Kl. Abt. 1. 121 : 667-705. 1912. 



