SEED GERMINATION 539 



Another important point is Gassner's opinion that the Ught effect is 

 localized in the coat. This idea is strongly contested by Lehmann and 

 his school, who beUeve that the inner living part of the seed is affected 

 by light, and not the coat. 



According to Lehmann (1913), for example, light acts catalytically 

 upon the living seed content. He bases this opinion first of all on the 

 fact that the laws of stimuli are valid for photoblastism and that these 

 laws apply only to living matter. Crocker (1929) remarks very rightly 

 that the product law is valid only in very narrow limits and that it is 

 possible that within such limits it could apply to nonliving tissues as 

 well as to living matter. It may be added that it is vaUd even for the 

 photographic plate. 



But there are other facts showing that the light effect may be in part 

 upon the embryo and not the coat. Only two will be cited. Leggatt 

 (1948) produced photodormancy in lettuce by exposing the seeds to blue 

 light. He then dried the ungerminated seeds, pricked the coats of some, 

 and germinated pricked and unpricked seeds in light and darkness. In 

 light, pricked and unpricked seeds germinated ecjually well; in the dark, 

 unpricked seeds had a very low germination percentage, and the germi- 

 nation of the pricked seeds was retarded. If the blue light effect had 

 been primarily upon the coat, equal germination of the pricked seed in 

 light and darkness would have been expected. Bihlmeier (1927) reports 

 for Nicotiana that those decoated seeds which did not germinate in dark- 

 ness germinated to about 30 per cent when exposed to light. 



But, wherever the light effect may be localized, what is its nature? 

 Gassner leaves this cjuestion more or less open. Most authors describe 

 the light effect as "photochemical." This opinion is supported by the 

 observation of Kincaid (1935) that with Nicotiana the light effect is inde- 

 pendent of the temperature at the time of the exposure to light. Gardner 

 (1921) is of the opinion that light activates lipase in the seed coats of 

 Rumex crispus. The enzyme then hydrolyzes the lipoids of the coat, 

 thus making them more permeable. But most authors think that the 

 light effect is in some way related to oxidation-reduction phenomena 

 (Axentieff, 1929; Bohmer, 1928; Kipp, 1929; Leggatt, 1948). Light 

 stimulates or inhibits oxidation processes during germination (Axentieff", 

 1929). As for positively photoblastic seeds, the oxygen optimum for 

 germination lies around 20 per cent. Bohmer (1928) thinks that, up to 

 this optimum, light and increasing oxygen content stimulate germination, 

 since the oxygen taken up is needed for respiration. When the oxygen 

 content is further increased, the oxygen taken up is not used for respi- 

 rational processes and starts processes antagonistic to the light effect. 

 Then increasing amounts of light are needed to counteract the inhibiting 

 effect of rising amounts of oxygen. For negatively photoblastic seeds 

 the order is reversed: oxygen above the amount needed for respiration 



