210 STRUCTURAL BOTANY 



took place. It is not necessary that the plants should 

 appear green externally, it is enough that they con- 

 tain chlorophyll For instance, the leaves of the 

 Copper Beech are of a brownish-red colour, yet they 

 assimilate like ordinary green leaves, because they 

 contain chlorophyll, which, however, is disguised by 

 the presence of a red colouring matter in addition. 



We already know that chlorophyll is contained in 

 the protoplasmic chlorophyll-corpuscles or plastids. 

 The whole apparatus is necessary for the work to be 

 done; the chlorophyll is useless for the purpose of 

 assimilation without the protoplasmic plastid, and the 

 plastid cannot assimilate without the chlorophyll. 

 We are not at all clear at present as to the special 

 duties performed by the two partners. Most probably 

 the chlorophyll serves to catch certain rays of light, 

 by the energy of which the plastid is enabled to do 

 the work of decomposing carbon dioxide and water. 

 It has been proved that the red rays of light are the 

 most active in promoting carbon-assimilation. The 

 absorption spectrum of chlorophyll shows a conspicu- 

 ous dark band in the red. Hence we know that 

 these red rays are absorbed by the chlorophyll, and 

 do not pass through it as light. It is very probable 

 that it is the energy of these absorbed rays which is 

 employed on the work of assimilation. 



We have now seen that carbon-assimilation is 

 dependent on light ; it is also dependent on tempera- 

 ture. Although all other conditions may be favourable, 

 yet if the temperature be too low no assimilation takes 

 place. Thus, in our experiment with the Elodea, we 



