252 LECTURE XIII. 



phyll : it is evident, therefore, that green plants must be 

 largely supplied from without with kinetic energy in some 

 form or other. 



We have already noticed more than once that the meta- 

 bolic processes of plants are materially affected by external 

 conditions, especially by the presence or absence of light, and 

 by variations in the temperature of the surrounding medium. 

 A somewhat elevated temperature is, in fact, essential to the 

 active life of all plants, but light is essential only to the 

 life of those plants which contain chlorophyll. This naturally 

 suggests that the energy requisite for the life of plants is 

 obtained by them either in the form of heat or of light. 

 With regard to heat, its importance is not that it affords a 

 continuous supply of energy to be converted into work in the 

 plant, but that it determines the initiation of chemical pro- 

 cesses which are carried on by means of energy obtained from 

 other sources : hence the supply of energy in the form of 

 heat is relatively small, as compared, on the one hand, with 

 the supply of (potential) energy afforded by their food to 

 plants which do not possess chlorophyll, and on the other 

 hand, with the supply obtained in the form of light by plants 

 which do possess chlorophyll. With regard to light, we 

 know of a mechanism in plants, but only in plants possessing 

 chlorophyll, by which the radiant energy of the sun's light 

 is converted into work : light then is the special form in 

 which kinetic energy is supplied to green plants. But in 

 addition to its importance in the constructive metabolism of 

 green plants, light has, as we shall see, a modifying influence 

 upon certain of the metabolic processes, the nature of which 

 is not in all cases perfectly understood. We will now study 

 in detail the relation of light . and heat to the metabolism of 

 plants. 



Light We learned in a previous lecture (p. 157) that a 

 green plant is incapable of constructing organic substance 

 from the materials of its food unless it is exposed to light : 

 and not only does it not increase in weight, when in darkness, 

 but it loses weight in consequence of the exhalation of carbon 

 dioxide and aqueous vapour in respiration (p. 195). Prolonged 



