52 LECTURE IV. 



outward conditions under which the carbonic acid assimilation takes 

 place. We know that cells containing chlorophyll are absolutely indis- 

 pensable to the process. In order to disturb the condition of equilibrium 

 in the carbon dioxide molecule and in those of the water required to form 

 the assimilation products of carbon dioxide in each of these compounds 

 the affinity of carbon and hydrogen for oxygen is completely saturated 

 energy is necessary. The plant cells perform work in transforming 

 kinetic energy into potential energy. It has been known for a long time, 

 and proved experimentally, that cells containing chlorophyll are not of 

 themselves capable of assimilating carbonic acid. The process takes 

 place only by the aid of light. The light vibrations of the ether furnish 

 the energy. All the rays of white light are not active in this respect. 

 In fact, the so-called chemical, or actinic rays, of the ultra-violet and 

 similarly the peculiar heat rays of the infra-red part of the spectrum 

 have little or no power of furnishing the cells with the energy necessary 

 for assimilation. The most active rays are the red, orange, and yellow. 1 

 These relations have been established by the work of Engelmann. 2 

 The essential moment, therefore, in the formation of organic substances 

 in the plant cell is the transformation of radiant energy into chemical 

 energy, a process which, as far as we know, takes place exclusively in 

 cells containing chlorophyll. Chlorophyll takes part, thereby, in an 

 important phase in the energy cycle. By its help, kinetic energy is trans- 

 formed into potential energy, and when later the plant is eaten by the 

 animal, this is eventually changed back into kinetic energy. Yet even in 

 the vegetable world the last-mentioned process plays an important part, 

 for we know of whole groups of plants which are not able to assimilate 

 carbon dioxide themselves ; these are the parasites which contain no chloro- 

 phyll, and, as regards their metabolism, they are closely related to the 

 animal organism; while on the other hand, we have species belonging to 

 the animal kingdom (vorticella, certain Flagellata, planarians, hydra, etc.), 

 which assimilate carbonic acid, and set free oxygen, thus imitating plants 

 in their metabolism. It has been found that the assimilation of carbonic 

 acid in such cases is also due to the presence of the same agent, chlorophyll, 



1 The maximum assimilation for the bluish-green, fresh-water algae and the red sea- 

 algae is effected by rays in other parts of the sun's spectrum. Engelmann has shown 

 (loc. cit.} that the light rays act more strongly in proportion as they are absorbed by 

 the color. In general, light complementary to the color of the plant is most active upon 

 assimilation. Spectroscopic analysis shows that of light passing through different 

 depths of water, the red rays are strongly absorbed, while the green and bluish-green 

 are less so. This explains why at greater depths of water, the red and yellow forms 

 prevail rather than those which are blue or bluish-green in color. Cf. W. Engelmann: 

 Arch. Physiol. Suppl. 1902, p. 333. Gaidukow: ibid. p. 214. 



2 Th. W. Engelmann: Bot. Ztg. 1882, 419; 1883, 1, 1884, 80; 1886, 64; 1887, 393, 

 Pfliiger's Arch. 26, 285 (1881); ibid. 26, 537 (1887); 27, 485 (1882); 30, 95 (1883). 



