254 LECTURE XIII. 



The reason of this is that the food, consisting as it does largely 

 of complex organic substances, supplies, on being decomposed 

 in the body, the necessary energy for the carrying on of the 

 assimilative or constructive processes. Such a plant is there- 

 fore independent of any supply of kinetic energy from without, 

 except in so far as it is dependent upon an adequate tempe- 

 rature for the initiation of the decomposition of its organic 

 food. A plant which does possess chlorophyll is incapable 

 of constructing complex organic substances out of the ma- 

 terials of its food in darkness, for it then has no supply of 

 energy by means of which the necessary chemical processes 

 could be effected. It can only do this when it is exposed to 

 light, and can absorb the necessary radiant energy. It is 

 true that a green seedling or a shoot can live for a time in 

 darkness, and also increase in weight, but it does so, not by 

 assimilating food-materials, but at the expense of complex 

 organic reserve-materials stored up in some depository with 

 which it is in connexion. Thus, a shoot may grow from a 

 potato-tuber to a great length in the dark, but this is accom- 

 panied by a decrease in absolute weight ; that is, that the dry 

 weight of the shoot and of the tuber, taken together, is less 

 than the original dry weight of the tuber. The same is true, 

 as shewn above, of seeds and seedlings. 



In a previous lecture (ix. p. 1 57) it was pointed out that it 

 is by means of their chlorophyll that green plants are enabled 

 to avail themselves of the kinetic energy of the sun's rays ; we 

 will now discuss this point more fully than we did then. It 

 was then mentioned that there are two principal conflicting 

 views as to which of the rays of the spectrum are the most 

 efficacious in promoting the decomposition of carbon dioxide 

 by the chlorophyll-corpuscles of plants, that, namely, of 

 Draper and of Pfeffer, according to which the yellow rays are 

 the most active, and that of Lommel, Timiriaseff, and others, 

 according to which it is the rays which correspond to the 

 most conspicuous absorption-band (band I, see plate) of the 

 chlorophyll-spectrum, the rays, that is, between the lines B 

 and C of the solar spectrum, at the junction of the orange and 

 of the red, which are the most active. The obvious difficulty 



