292 VEGETABLE PHYSIOLOGY 



being generally very poor conductors of heat. Still it 

 seems not improbable that a certain amount of energy is 

 devoted to the attainment of the range which is most 

 suitable for the vital processes. Though the dominating 

 factor in the determination of the plant's temperature is to 

 be looked for in the environment, the occurrence of the 

 warmth of germination and of the expansion of the flower- 

 buds is an indication that it is not the only one. 



A fuller consideration of the relations of the plant to 

 heat must be deferred to a subsequent chapter. 



The evolution of light by plants is a comparatively rare 

 phenomenon, being probably confined to certain Fungi, 

 though it has been attributed also to a few species of 

 Algae. It must call, however, for a certain expenditure of 

 energy in such cases as have been authenticated. 



If we turn now to consider the sources of this energy, it 

 is evident that they must be in the first instance of external 

 origin. The radiant energy of the sun indeed is the only 

 possible source which can supply it to normal green plants. 

 The question of the absorption of this energy has already 

 been incidentally alluded to when we discussed the chloro- 

 phyll apparatus, but it may now be examined more closely. 



The rays which emanate from the sun are generally 

 alluded to as falling into three categories, those of the 

 visible spectrum, those of the infra-red, and those of the 

 ultra-violet. The second of these are frequently spoken 

 of as heat rays, and the last as chemical. 



The greatest absorption of energy appears to take place 

 in consequence of the peculiarities of chlorophyll. As we 

 have seen, this substance, whether in the plant or when in 

 solution in various media, absorbs a large number of rays 

 in the red and in the blue and violet regions of the 

 spectrum, together with a few others in the yellow and the 

 green. 



The solar spectrum after the light has passed through 

 a solution of chlorophyll is seen to be robbed of rays in 

 these regions, and hence to present the appearance of a 



