818 BIOLOGICAL EFFECTS OF RADIATION 



the seeds to light during examination may have been sufficient to force 

 the germination of the checks in darkness. The species and varieties of 

 tobacco seeds also vary considerably in Ught sensitiveness. Lehmann 

 (64) showed that an exposure of 0.1 sec. of imbibed Lythrum salicaria 

 seeds to 730 H. K. of light at 30°C. gave 50 per cent germination in 24 hr. 

 against 6 to 7 per cent in darkness. Seeds of Nicotiana tahacum and 

 Lythrum salicaria appear to be the most sensitive to small dosages of 

 Ught of any seeds studied to date. 



This great variation in the Ught dosage required by light-sensitive 

 seeds suggests the possibility that the mechanics of light action may be 

 different for different seeds. 



THEORIES OF LIGHT ACTION 



Many of the theories of light action on seeds have already been 

 discussed in connection with the work of the various investigators. The 

 stimulus or releasal theory of Pfeffer and Jost has little support in the 

 present stage of physiological investigation, especially since the findings 

 of Went (100) and his students on "Wuchsstoff" (auxine) have placed 

 phototropism and geotropism, two strongholds of the stimulus conception, 

 upon a chemical basis. 



In his early studies of Ught-favored seeds Cieslar (12) suggested 

 several possible explanations of light action. Since yellow light proved 

 so effective, he thought C assimilation might occur. This is hardly 

 possible because no chlorophyll is present in most seeds favored and 

 because, as Heinricher later (33) showed, yellow light stimulates germina- 

 tion in both the presence and absence of CO2. He also suggested the 

 possibiUty of increased osmotic pressure in the tissues, or of mobilization 

 of reserves, the theory which Heinricher (33, 35) later adopted. This 

 conception could be tested experimentally, but it lacks sufficient evidence 



to date. 



Lehmann (62) at first accepted the PfefTer and Jost releasal theory, 

 but later (63, 68) expressed the view that Ught acted catalytically, and 

 concluded that hydrolysis of storage proteins was the effective change. 

 His strongest evidence for protein hydrolysis was the fact that soaking 

 various light-favored seeds in proteolytic enzymes substituted for light. 

 He thought acids either activated proteolytic enzymes or gave a more 

 favorable pH for their action. Lehmann's whole theory lacks experi- 

 mental proof. It is doubtful whether the large molecules of the enzymes 

 pass through the semipermeable membranes of the seed coats. It may 

 be that the proteolytic enzymes furnish nitrogen which acts on the coats, 

 as Gassner found for nitrates. 



Pickholz (84), working on Poa achenes, concluded that light had its 

 main effect through the production of daily intermittent temperatures. 

 The facts that direct sunlight is more effective than other light sources 



