THE ASSIMILATION OF CARBON BY AUTOTROPHIC PLANTS. II 131 



phyll acts as a sensitizer. It is well known, for instance, that silver salts are 

 affected only by rays of a certain wave length, and that red light, as every 

 photographer knows, has no effect on them. By adding a colouring matter 

 which absorbs red light, the silver salts become sensitive to red rays. The 

 action of the colouring matters in this relation is by no means clearly under- 

 stood, since not all dyes which absorb red rays act as sensitizers. Apart from 

 that, there is one great distinction between these physical peculiarities and those 

 presented by the chlorophyll body, viz. that silver salts are naturally affected 

 by light, and their sensitivity is only increased by employing a sensitizer ; 

 the chlorophyll body, on the other hand, is unable to bring about the decom- 

 position of carbon-dioxide in the absence of the green colour, so that we cannot 

 look upon the dye merely as a sensitizer. 



Seeing that the manufacture of organic material is necessarily bound up 

 with the entry of energy into the plant, it may be asked whether this energy is 

 of necessity always solar energy, and more especially that of the luminous 

 rays. It might well be supposed that other forms of energy also, heat, elec- 

 tricity, and chemical energy, might be employed for the same purpose, and, as 

 a matter of fact, it is very probable that some organisms can construct organic 

 material out of inorganic with the aid of chemical energy. This mode of con- 

 struction may be termed chemosynthesis in contrast with photosynthesis — the 

 process we have already been studying (Pfeffer, Phys. 2nd ed.). We shall 

 recur to chemosynthesis later on, but at the present moment, in view of the 

 importance of ' photosynthesis ', a few remarks as to the history of the subject 

 will not be inappropriate. (Compare Sachs, 1875 ; Pfeffer, Physiol. 2nd ed. 

 I, 289 ; Brown, 1899 ; [Wiesner, 1905].) 



The essential preliminary basis on which this theory of carbon assimilation 

 is built was laid down in a series of researches published in the last third 

 of the eighteenth century. Priestley was aware that the atmosphere was 

 rendered foul owing to the respiration of animals and to putrefaction and com- 

 bustion, and he strove systematically to discover by what means nature counter- 

 acted this. In 1771 he established the fact that this duty was fulfilled by the 

 plant world. It was he who in 1778 first made out that the air-bubbles escaping 

 from partly submerged plants contained more oxygen than ordinary air. In 

 •the glass vessels which he employed in his researches he noticed, after long 

 standing, the development of green masses which also gave off oxygen in 

 sunlight, but since Priestley was unacquainted with the fact that these 

 green masses were Algae, he thought he was observing a purely chemical process 

 tending to the evolution of oxygen. Priestley" does not seem to have 

 clearly appreciated the importance of sunlight in the ' purification of the 

 air', and it was Ingenhouss who first drew attention to the fact that it 

 was the green parts of plants that alone had the power of inducing this 

 'purification'. Both Ingenhouss and Priestley were supporters of the 

 phlogiston theory. Senebier was the first to investigate the subject from 

 the standpoint of modern chemistry as founded by Lavoisier, and his 

 exposition of the subject appears to us to-day to be much more modern than 

 those of his predecessors. He showed more especially the connexion between 

 the carbon -dioxide and the evolution of oxygen, and was the first to make out 

 that it was concerned in the process of the manufacture of organic material. 

 It is true that, knowing how little carbon-dioxide was present in the air, he 

 thought that plants absorbed the gas from the soil. Th. de Saussure (1804), 

 however, first brought forward incontestable evidence that the air was the 

 source whence the plant obtained its carbon-dioxide, and it was he who, by his 

 exceedingly accurate experiments, placed our entire knowledge of the subject 

 on a sound basis. Later on, his correct interpretation of the facts was 

 neglected and ' humus ' was once more thought to be of importance in the 

 nourishment of the green plant, until, through Liebig's sagacity and Bous- 



