THE ENEKGY OF THE PLANT 285 



the same temperature as the medium in which it lives, 

 though the process of adjustment is often very slow, the 

 tissues 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 suit- 

 able 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 development of heat 

 during germination and while the flower-bud is opening 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 the plant's 

 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 be of service to normal 

 green plants. The question of the absorption of this energy 

 has already been incidentally alluded to when we discussed 

 the chlorophyll 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 



