470 



SCIENCE. 



[N. S. Vol. XII. No. 300. 



coherent theory of nutrition ; such as it 

 was it amounted to little more than the 

 conclusion arrived at by van Helmont a 

 century and a half earlier, that plants re- 

 quire only water for their food, and are 

 able to form from it all the different constit- 

 uents of their bodies. It is true that the 

 important discovery had been made and 

 pursued by Priestley (1772), Ingen-Housz 

 (1780), and Senebier (1782) that green 

 plants exposed to light absorb carbon 

 dioxide and evolve free oxygen ; but this 

 gaseous interchange had not been shown to 

 be the expression of a nutritive process. 

 At the opening of the nineteenth century 

 (1804) this connection was established by 

 de Saussure, in his classical ' Recherches 

 chimiques,' who demonstrated that, whilst 

 absorbing carbon dioxide and evolving 

 oxygen, green plants gain in dry weight ; 

 and he further contributed to the elucida- 

 tion of the problem of nutrition by show- 

 ing that, whilst assimilating carbon dioxide, 

 green plants also assimilate the hydrogen 

 and oxygen of water. 



Three questions naturally arose in con- 

 nection with de Saussure's statement of the 

 case : What is the nature of the organic 

 substance formed? What is the function 

 of the chlorophyll? What is the part 

 played by light? It was far on in the 

 century before answers were forthcoming. 

 With regard to the first of these questions, 

 the researches of Boussingault (1864) and 

 others established the fact that the volume 

 of carbon dioxide absorbed and that of 

 oxygen evolved in connection with the proc- 

 ess are approximately equal. Further, 

 the frequent presence of starch in the 

 chloroplastids, to which Mohl first drew 

 attention (1837), was subsequently found 

 by Sachs (1862) to be closely connected 

 with the assimilation of carbon dioxide. 

 The conclusion drawn from these facts is 

 that the gain in dry weight accompanying 

 the assimilation of carbon dioxide is due to 



the formation, in the first instance, of or- 

 ganic substance having the composition of 

 a carbohydrate ; a conclusion which may 

 be expressed by the equation : 



CO, + H,0 = CH,0 + 0,. 



The questions with regard to chlorophyll 

 and to light are so intimately connected 

 that they must be considered together. 

 The first step towards their solution was 

 the investigation of the relative activity of 

 light of different colors, originally under- 

 taken by S6n6bier (1782) and subsequently 

 repeated by Daubeny (1836) , with the result 

 that red and orange light was found to pro- 

 mote assimilation in a higher degree than 

 blue or violet light. Shortly afterwards 

 Draper (1843), experimenting with an 

 actual solar spectrum, concluded that the 

 most active rays are the orange and yellow ; 

 a conclusion which was generally accepted 

 for many years. But in the meantime the 

 properties of the green coloring matter of 

 plants (to which Pelletier and Caventou 

 gave the name ' chlorophyll ' in 1817) were 

 being investigated. Brewster discovered 

 in 1834 that an alcoholic extract of green 

 leaves presents a characteristic absorption 

 spectrum ; but many years elapsed before 

 any attempt was made to connect this 

 property with the physiological activity of 

 chlorophyll. It was not until 1871-72 that 

 Lommel and N. J. C. Miiller pointed out 

 that the rays of the spectrum which are 

 most completely absorbed by chlorophyll 

 are just those which are most ef&cient in 

 the assimilation of carbon dioxide. Sub- 

 sequent researches, particularly those of 

 Timiriazeff (1877), and those of Engelmann 

 (1882-84) based on his ingenious Bac- 

 terium-method, have confirmed the views 

 of Lommel and of Miiller, and have placed 

 it beyond doubt that the importance of light 

 in the assimilatory process is that it is the 

 form of kinetic energy necessary to effect 

 the chemical changes, and that the function 



