810 BIOLOGICAL EFFECTS OF RADIATION 



Lehmann (64) found that exposure of imbibed Lythrum salicaria seeds 

 to 730 H. K. of light for 0.1 sec. at 30°C. gave 50 per cent germination 

 in 24 hr., against 6 to 7 per cent without the exposure after 10 days. 

 Lehmann (65) and Lehmann and Lakshmana (67) showed that a relation 

 existed between the germination and the product of hght intensity and 

 time of the exposure. This law applied fully at 31°C. with intensities 

 of 5, 50, and 100 M. K., but there was considerable deviation with lower 

 temperatures or higher intensities of Hght. The deviations from the law 

 were even greater when temperatures of 25°C. or lower and high hght 

 intensities were combined. Within Hmits the Talbot law also applied, 

 although in cases where the dark periods between exposures were long, 

 the individual periods of illumination proved excessively effective. 



Lehmann concluded that since these two laws applied, the action of 

 light was upon the protoplasm. One must remember, however, that the 

 product law applied only within narrow limits, and the Talbot law showed 

 one limitation. In C Moris Gassner thought that Hght acted on the coat, 

 while Wieser believed both effects possible. It may be that for some 

 seeds the action of light is upon the coats, for others upon the protoplasm, 

 and for still others upon both the coats and protoplasm. 



Wieser (102) found that different samples of L. salicaria seeds varied 

 greatly in their need of Hght for germination, and that only imbibed 

 seeds were affected by light. He also observed the latent Hght effect 

 in these seeds, as Gassner had in Chloris and Ranunculus achenes. He 

 concluded that the variation in Hght need shown by different samples of 

 seeds might be due to the variation in the amount of light exposure the 

 seeds received during ripening in the capsules previous to drying out. 



Various investigators (16, 25, 41, 65, 68, 81, 101) agreed that weak 

 acid solutions favored the germination of L. salicaria seeds in darkness, 

 especially if combined with moderately effective intermittent tempera- 

 tures. Gassner grouped Lythrum with the acid-type of Hght-favored 

 seeds. Nitrogen compounds (41, 81) increased germination in darkness. 

 Increased partial oxygen pressure (8, 102) did not increase the germina- 

 tion in light. Wieser found that light forced germination only during 

 normal respiration and not during intramolecular respiration. 



Other Seeds Favored by Light. — The sections above cover the light- 

 favored seeds that have been studied in greater detail. There are a 

 number of other seeds that have received considerable attention, but 

 their consideration will not add anything in fact or principle that will 

 further an understanding of the subject. It might be well, however, to 

 summarize briefly the extensive work of Kinzel (51). He studied seeds 

 or fruits of 964 species of plants as to their sensitiveness to light. He 

 used rather low temperatures, 17° to 20°C., and did not try most of the 

 conditions that other investigators found effective as substitutes for light. 

 Because of the restricted conditions under which he made these studies 



