528 RADIATION BIOLOGY 



nation with light or alternating temperatures (Lehmann, 1924; Fassben- 

 der, 1925). 



Hesse (1924) classifies the photoblastic seeds into two groups: 



1. The obligate nitrogen type, where only nitrogen compounds bring 

 about germination in darkness (e.g., Chloris ciliata, four species of 

 Epilohium.) . 



2. The facultative nitrogen type, where nitrogen compounds and acids 

 promote germination (e.g., Lythrum salicaria, two species of Verbascum). 



It is interesting that, with one exception, negatively photoblastic seeds 

 are not influenced in their germination by acids or nitrogen compounds 

 (Bohmer, 1928). 



The following substances, too, stimulate germination of positively 

 photoblastic seeds in the dark: ether, saponin, digitonin (Niethammer, 

 1928), papayotin, trypsin (Lehmann, 1913; Lehmann and Ottenwalder, 

 1913). Special mention must be made of coumarin. If nonphotoblastic 

 lettuce seeds are treated with this compound, they are made positively 

 photoblastic (Nutile, 1943-1944, 1945). Many other compounds not 

 chemically related to coumarin and to each other, such as parasorbic 

 acid and sucrose, have the same effect (Weintraub, 1948). 



Naturally Occurring Inhibitors. A relation exists between naturally 

 occurring inhibitors and the effect of light upon germination (Evenari, 

 1949) . Seed extracts from Phacelia inhibit germination to a higher degree 

 in Hght than in darkness (Magnus, 1920; Peters, 1924; Bohmer, 1928). 

 Extracts of Pelargonium leaves (Magnus, 1920) and of Phacelia and 

 Pisum, for example, have the same effect. 



Extracts of seedballs of beet strongly inhibit the germination of beet 

 seed and of a number of nonphotoblastic seeds such as Avena (Duym 

 et al., 1947), Gypsophila, and Linum (Froschel, 1940) much more in light 

 than in darkness; i.e., they make these hght-indifferent seeds negatively 

 photoblastic. 



Gases. The photoblastic reaction is dependent not only upon pretreat- 

 ment with different gases (see Sect. 3-2) but also upon the gases present 

 during illumination and germination. It was found for three negatively 

 photoblastic seeds {Phacelia, Nigella, Amaranthus) that with increasing 

 partial pressures of oxygen the inhibition of germination caused by light 

 is greatly reduced (Bohmer, 1928; Resiihr, 1939a). Positively photo- 

 blastic seeds {Epilobium, Lythrum, Nicotia7ia, Eschscholzia, Plantago) , when 

 germinated in atmospheres of different oxygen content and illuminated 

 only for some minutes, are inhibited in their germination by increasing 

 oxygen content (ibid.). 



This inhibition is overcome by higher light intensities (Resiihr, 1939a). 

 When continuous illumination is apphed, there is no difference in germi- 

 nation for the different oxygen contents (Bohmer, 1928). 



The same experiment in darkness gives different results for different 



