538 RADIATION BIOLOGY 



are positively photoblastic. Dehiilled grains do not require light for 

 their germination, since the free entry of oxygen is ensured by the absence 

 of the hulls. Dehulled grains are made positively photoblastic by insuf- 

 ficient oxygen supply, nonoptimal temperatures, and insufficient after- 

 ripening. All three factors cause, in Gassner's opinion, a decrease of the 

 germination energy, i.e., a retardation of germination. During the germi- 

 nation of Chloris two different processes occur side by side: one process 

 typical for the germination of all seeds, photoblastic and nonphotoblastic 

 alike, and another one typical for photoblastic seeds only. This process, 

 according to Gassner, consists in the formation of an "inhibiting layer" 

 or "inhibiting principle" inside the fruit or seed coat. This inhibiting 

 layer does not exist beforehand but is formed during the initial stage of 

 germination. In darkness there is nothing to counteract the formation 

 of this layer, so that there is no germination in the dark. The desensibili- 

 zation processes, leading to skotodormancy, are, according to Tilly (1934- 

 1935), an indication of the formation of this inhibitory layer, which is 

 destroyed by nitrates, acids, and other chemicals (Hesse, 1924). Light 

 inhibits or retards the formation of this layer; i.e., light acts on the coats, 

 inside which certain chemical processes bring about the formation of the 

 inhibiting layer. Insufficient oxygen supply, nonoptimal temperatures, 

 or insufficient afterripening inhibits germination in the dark as a result 

 of a decrease in the germination energy and the formation of the inhibiting 

 layer before the radicle can penetrate the seed coat. When light is pres- 

 ent, this retardation of germination does not matter, since the light 

 inhibits or retards the formation of the inhibiting layer. 



The first important point in this theory is the role played by the hull 

 or the fruit and seed coats. Most authors agree that the coats interfere 

 with the gas exchange necessary for normal germination. There can be 

 no doubt that the coats deeply modify the nature of seed respiration 

 (e.g., Frietinger, 1927). This explains why the removal of the coats 

 brings about germination under conditions that do not allow germination 

 of the intact seed. But this inhibition of normal gas exchange may be 

 an inhibition of oxygen entry into the seed (Gassner, 1911a, b; Becker, 

 1913; Davis, 1924; Axentieff, 1929), or it may be an inhibition of carbon 

 dioxide exit from the seed into the atmosphere (Leggatt, 1948). The 

 inability of the intact seeds to germinate would be explained in the first 

 case by the lack of the necessary oxygen and in the second case by the 

 presence of carbon dioxide, which produces dormancy (ibid.). We may 

 add here that Forward (1949) found for oats that carbon dioxide brings 

 about dormancy and that the dormant condition of freshly harvested 

 grains is caused by ac(nimulation of carbon dioxide. This dormancy is 

 broken by any method that either sets the carbon dioxide free (pricking 

 of coats) or increases the concentration of a substance (oxygen) that 

 counteracts the carbon dioxide effect. 



