Environment and Adaptation 209 



and the degree of light often exercises a strong 

 influence upon the production of the reproductive 

 organs of many algae. Many flowering plants, also, 

 growing with insufficient light, develop few or no 

 flowers. 



Nature of the Light Stimulus. The nature of 

 the light stimulus is very obscure, and it is not likely 

 that it is always the same. In some cases, e.g., 

 where certain green spores refuse to germinate in 

 darkness it is quite probable that this is on account 

 of the failure to develop certain products of photo- 

 synthesis before germination can begin, and this is 

 made the more probable, as sometimes by supplying 

 sugar, which might very well replace some of the 

 products of photosynthesis, moss-spores may be 

 made to germinate in darkness. The stimulus of 

 light is not, however, indispensable in all cases, as 

 many spores normally germinate in darkness, and 

 apparently can develop the necessary stimulus for 

 growth without the aid of light. The direct effect of 

 light upon the rate of growth is usually a retarda- 

 tion. Plants growing in darkness become exception- 

 ally elongated, and there is also a difference in the 

 size of the leaves growing in shade and in full light. 

 How far the larger size of the shade leaf is the 

 direct effect of the action of the diminished light, 

 and how far it is only a correlation, bound up with 

 the necessity for a greater exposure of green tissue 

 owing to the diminution of photosynthesis, it would 

 be hard to say. 



