346 Comparative Physiology. 



the one producing and reacting on the other. It has been said, 

 light acts chiefly by its chemical power, but the chemical rays 

 are at the faintest end of the spectrum, and possess less mo- 

 mentum than the red ; this is said to cause the azure -blue colour 

 of the sky, by their small momentum causing them to be re- 

 tained, and the red colour of the rising sun by the red rays 

 possessing most momentum, and being first seen. Faint blue 

 light, however, does not produce so much effect on vegetation, 

 which is always greater the whiter and more intense the light. 

 It is likely, therefore, the intense sunbeam containing both 

 light, heat, and electricity, is the best adapted to encourage 

 vigorous growth. Faint light will produce action, as is seen in 

 the action of the moonlight on plants, especially in tropical 

 countries ; but white light is most powerful. Light is not in- 

 dispensable to vegetation, as may be seen in bulbs and tubers 

 produced at roots without the foliage ever having been above 

 ground, if any were produced. Miiller says, it is a general rule 

 in organic action, that many various stimulants are capable of 

 exciting the organs to produce their action, and still the action 

 will be produced in the same normal form. It is likely that 

 the heat, light, and electricity of tlie sunbeam are all con- 

 cerned in furthering organic action. Plants differ greatly in 

 their power of being affected by light ; perhaps from their being 

 too susceptible of stimuli, it is too powerful for some, as the 

 leaves are found to get feeble and flaccid when exposed long 

 to full light and heat. The power of standing much stimulus 

 appears also to depend on the power of absorption ; such leaves 

 as those of the camellia would appear not so susceptible of being 

 hurt by stimulus as those of the geranium ; yet, from the more 

 vigorous roots of the latter, it is enabled to stand much more 

 light than the former, which thrives better in heat and dif- 

 fused light. Alpine plants, being naturally much exposed to 

 light, should thrive well in its presence ; they are accordingly 

 found to like it in winter : but in summer the heat of the sun- 

 beam is too powerful, and they require shading with all the air 

 possible; the cool air sifted through a hedge appears to suit 

 them well. The green colour has been said to depend on light, 

 but it rather seems to depend on the alkaline state of the chro- 

 mule, which the deoxidising power of light fui'thers. In mines 

 where hydrogen abounds, plants are found green though shut 

 out from light ; probably through the formation of ammonia from 

 the hydrogen absorbed by the leaf uniting to the nitrogen of 

 the air deprived of its oxygen in the air vessels, and producing 

 thus the alkaline or green colour from the predominance of 

 alkali (ammonia), by a different process from that of deoxidation, 

 which is a diminution of acidity. Dumas describes the power 

 of light as being stored up in the leaf. Light appears capable of 

 being retained in a latent form. The experiments of Mr. Hunt, 



