LIGHT. 191 



in complete darkness, but they do so at the expense of ma- 

 terial which has beeii elaborated in light. So, too, sonic 

 parts of many ordinary plants grow in total darkness, as 

 roots, tubers, bulbs, etc., but these depend for their carbo- 

 hydrates upon the aerial, chlorophyll-bearing parts which 

 are in the light. As will be shown in the sequel, this depen- 

 dence of all life upon light is due to its relation to chloro- 

 phyll in the processes of assimilation ; and while other func- 

 tions than that of assimilation and other orgars than those 

 which contain chlorophyll are somewhat affected by the 

 presence or absence of light, or its greater or less intensity, 

 yet these latter are of comparatively little moment when com- 

 pared with the former. 



The absorption of water by the plant appears to be entirely 

 independent of light, and in most plants it takes place in its 

 entire absence. Likewise it is probable that light itself does 

 not directly affect the rate of evaporation of water from the 

 leaves of higher plants. As, however, the stornata are gen- 

 erally opened more widely in light than in darkness, evapo- 

 ration may be promoted by it in some cases. 



250. Light and Assimilation. It is first of all to be 

 observed that chlorophyll itself is dependent upon light. 

 Those parts of plants (with rare exceptions) which grow in 

 darkness are destitute of chlorophyll, and even parts which 

 contain chlorophyll lose it when placed for some time in com- 

 plete darkness. When such a colorless plant is brought into 

 the light it soon becomes green from the formation of chlo- 

 rophyll in its protoplasm. 



The decomposition of carbon dioxide, and the consecpient 

 evolution of oxygen, only take place in the light. As the 

 light decreases in intensity from a certain point the amount 

 of assimilation decreases ; on the other hand, there is a de- 

 crease in assimilation as the intensity increases unduly, and 

 beyond certain points in either direction assimilation ceases. 

 Thus there are here, as in the case of temperature, a mini- 

 mum, optimum, and maximum ; but we cannot define their 

 limits as readily, for want of a proper instrument. 



251. Experiments have often been made upon plants 

 when placed in rays of different refrangibility, and it has 



