EFFECTS UPON GERMINATION 819 



and tliat intermittent temperatures in darkness are more stimulating than 

 liglit at any constant temperature point to this conchision for Poa. This 

 explanation, however, cannot apply to highly sensitive seeds like Nicoti- 

 ana and Lythrum, in which a single small dosage of light induces high 

 germination in darkness. 



From his investigations of Chloris achenes Gassner (23, 27) expressed 

 the opinion that light and factors substituting for light hindered the 

 gradual development of an inhibiting layer in the achene coats, either 

 directly or by favoring quick germination. The work of many investiga- 

 tors shows that seed and fruit coats play a significant part in the light 

 sensitiveness of several sorts of seeds and fruits and Gassner 's finding 

 that nitrates substitute fully for light in Chloris achenes without actually 

 entering the living parts of the achenes is very significant. It is possible, 

 however, that Hght and the several light-substituting factors promote 

 germination in a variety of ways either by acting on various portions 

 of the seeds or fruits or by acting differently on the same organs. A 

 careful chemical, microchemical, and physiological study of several light- 

 sensitive seeds and fruits is needed to learn the effect of light and its 

 substitutes on the hulls, coats, endosperms, or embryos. It would be 

 advantageous to investigate the isolated embryos of light-sensitive seeds 

 and fruits, as Flemion (19, 20) has done for seeds with dormant embryos, 

 to determine whether they are dormant and, if so, how various conditions 

 modify their growth. Gassner's hypothesis raises such definite questions 

 as whether nitrates favor the decomposition of the carbohydrates of the 

 hulls and coats by microorganisms through furnishing a nitrogen supply 

 for the microorganisms; and whether light and light-substituting factors 

 increase the activity of catalase or other enzymes of the embryos, or bring 

 about other changes that further the growth of the embryos. 



Kommerell (54) concluded that only the light passing through the 

 seed coats and reaching the protoplasm was effective and that the effect 

 was photochemical. Her theory and the facts and assumptions on which 

 it was based have been discussed already under Effect of Various Regions 

 of the Spectrum. 



Axentieff (3) recently offered an explanation of light sensitiveness 

 in seeds which takes into consideration both coat effects and the effect of 

 light upon the protoplasm of the seed contents. In order to determine 

 how generally coats modified light sensitiveness of seeds and fruits, the 

 author studied 8 species that were hindered by light and 4 species that 

 were favored. Of the seeds or fruits inhibited by light the following 

 showed the light-inhibiting effect to be entirely dependent upon the 

 presence and integrity of the coats: Amaranthus reiroflexus, Phacelia 

 tanacetifolia, Androsace maxima, and Bromiis squarrosus; and the follow- 

 ing showed that light-inhibiting effect was not entirely due to the coats : 

 Cucumis melo, C. sativus, Cucurhita pepo, and Nigella arvensis. In the 



