LIGHT AND GERMINATION 429 



activated by acids. Magnus (1920) showed that the ger- 

 mination of the " dark " seed of Phacelia, which takes place 

 in the Ught with acid, is " false," and is due to a mechanical 

 forcing of dead embryos from the seed coats. Hesse agrees 

 that this is so for most cases of acid activation. 



The net result of these investigations seems to be that 

 certain seeds under " ordinary " temperature conditions ger- 

 minate only in the light, and others only in the dark ; that, 

 in the former case an abnormally high, in the latter a low, 

 temperature makes the seed indifferent to light or dark ; that 

 in the inappropriate light condition germination may be 

 secured by treatment with nitrogenous compounds, or with 

 certain enzymes, or in some cases by removing the coats. 

 We may ask what is the mechanism of this relation to light. 

 Three theories have been put forward. 



{a) Lehmann holds that light acts in the " light " seed 

 by catalysing the conversion of reserve proteids into soluble 

 compounds ; this interpretation is supported by the action 

 of the proteolytic enzymes, and perhaps by the extremely 

 small light exposures which are required to give an improve- 

 ment in germination. In " dark " seeds the light is sup- 

 posed to activate fluorescent organic substances which 

 destroy the proteolytic enzymes. 



{b) Gassner analyses the conditions for Chloris into three 

 divisions : I, The embryo requires a certain after- ripening ; 

 II, the glumes act by preventing the access of oxygen, and 

 light has some effect in counteracting this ; III, light 

 further acts by destroying a substance inhibiting germina- 

 tion, which is produced in the seed coat in conditions of 

 temperature and moisture favourable to germination. 

 Gassner and Hesse apply this explanation, i.e. of inhibitors, 

 to light germination in general. It receives some support 

 from experiments of Magnus on the " dark " seed of 

 Phacelia. In low light intensities, germination to the 

 extent of about 30 per cent, of the seeds can be obtained ; 

 this is depressed by placing the seeds in water in which other 

 seeds have been soaked. The conclusion is drawn that an 

 inhibitor is present which acts only in light, or which 



