430 THE BIOLOGY OF FLOWERING PLANTS 



strengthens the action of light, in this case an inhibiting 

 one. 



(c) Crocker (19 16), in an extensive review of the whole 

 question of dormancy, suggests that light acts by altering 

 the condition of the seed coats in their relation to oxygen 

 and water supply. This view is supported by much of the 

 work of Gassner and Hesse. They have shown definitely 

 that the action of nitrogen compounds, which cause ger- 

 mination in the dark, is on the coats ; for concentrations, 

 e.g. of nitric acid, can be used which would be fatal if they 

 penetrated to the embryo, and which indeed are fatal if 

 applied to seeds with damaged coats. Treatment with 

 absolute alcohol, subsequently washed off, may also produce 

 dark germination of " light " seeds. Fluctuating tem- 

 peratures, and sudden rise in temperature, may well act by 

 causing a change in the colloids of the cell walls. The 

 germination in the dark of dark inhibited seeds, with the 

 coats removed, points in the same direction, and a similar 

 result has been obtained by Gardner (1921) for Rumex 

 crispus. At the same time it must be admitted that the 

 relations are so complex, and vary so much from species to 

 species, that we have at present no clear evidence decisively 

 in favour of any one of these hypotheses ; it is quite likely 

 that the mode of action of light or darkness may be different 

 in different cases, and that more than one effect may be 

 present. 



The biological aspect of the light relation resembles 

 that of other types of dormancy. We find again the necessity 

 of special conditions for germination, or, looking at it from 

 the other side, of special conditions in which dormancy 

 occurs. We have again the possibility of secondary dor- 

 mancy induced by the action of an external factor. Gassner 

 (19 10) has related the conditions in Chloris to the environ- 

 ment of the plant and we give his account, Chloris is a 

 typical summer grass of the South American pampas. 

 (" Summer " occurs in the months December to March.) 

 It flowers from December to March and fruits from January 

 to April. Throughout the cold season the seeds are 



