SEED GERMINATION 541 



photoblastic at higher temperatures, because light, besides retarding the 

 formation of the inhibiting layer, decreases the germination energy at 

 low temperatures. There are no facts to support his opinion. 



A better explanation is given in those cases in which low temperature 

 makes seed that are positively photoblastic at higher temperatures inde- 

 pendent of light (lettuce seed, for example). For these seeds Davis (1924) 

 has pointed out that low temperature decreases the oxygen requirement 

 of the seeds, which can then germinate in the dark with the restricted 

 oxygen supply that passes through the coats. There is naturally a fur- 

 ther possibility, i.e., that low temperatures increase the permeability of 

 the coats toward oxygen or carbon dioxide, and it may be that both 

 effects of low temperatures work together. We mention in this connec- 

 tion the findings of Brown (1940) that the permeability of Cucurhita 

 seed-coat membranes toward carbon dioxide was different at different 

 temperatures. 



An entirely different approach to the temperature effect, worthy of 

 close investigation, was given by Biinning (1948). He explains the dif- 

 ferent effects of light at different temperatures by supposing that the 

 stimulating and inhibiting processes produced by light are dependent 

 upon temperatures to varying extents. 



The fact that with afterripening photorequirement decreases and 

 photosensitivity increases could be explained by supposing that after- 

 ripening increases the permeability of the coats toward o.xygen and 

 decreases the oxygen reriuirements of the embryo. This was pointed 

 out by ShuU (1909, 1914) for Xanthium. Or it may be that by after- 

 ripening the relative amounts of inhibiting and stimulating photorecep- 

 tors are changed (Biinning, 1948). 



In reviewing all these different opinions brought forward to explain 

 photoblastism, we can only agree with Crocker (1936), who states that 

 the factual evidence for any one of these theories is entirely inadequate. 

 One of the reasons for this, besides the main one, i.e., the extremely com- 

 plex nature of the processes involved, is to be found in a fact pointed out 

 by Resiihr (1939b). When we speak about germination, we mean by 

 this the penetration of the seed coat by the radicle. At least three dif- 

 ferent seed parts have to join forces in order to bring about this process: 

 embryo, seed coat, and endosperm. If a seed does not germinate, we do 

 not know which part is responsible, since each one alone can be the 

 limiting factor. When we obtain the same percentage of germination, 

 it may be brought about in each case by a completely different relation 

 of the three factors involved. Without deeper knowledge of this, we 

 would jump to the conclusion that the obtaining of the same percentage 

 of germination in different experiments is due to the same factor— a con- 

 clusion that may not be true. 



