THE METABOLISM OF PLANTS. 141 



Senebier proved that the amount of-" pure air." (O) evolved by 

 green plants in water is greater when a considerable amount 

 of " fixed air " (CO 2 ) is held in solution. De Saussure 

 endeavoured to determine the relation existing between the 

 volume of carbon dioxide absorbed and the volume of oxygen 

 evolved, and found that the latter was smaller than the 

 former. He made, moreover, the important observation that 

 the decomposition of carbon dioxide by green plants under 

 the influence of light is accompanied by an increase in their 

 weight. 



Our knowledge on this subject at the beginning of the 

 present century may be briefly summed up as follows : green 

 parts of plants decompose carbon dioxide when exposed to 

 sunlight, and exhale a somewhat smaller volume of oxygen, 

 these processes being accompanied by an increase in weight. 



Of the various facts which are included in this summary 

 the one which especially concerns us now is the increase in 

 weight. Many years after the publication of de Saussure's 

 observations, von Mohl, in investigating the structure of 

 chlorophyll-corpuscles, noticed the almost universal occurrence 

 of starch-grains in them, and found that the starch-grains 

 are not constituent parts of the corpuscles, but are secondary 

 formations within them. These observations were subsequently 

 confirmed and extended by Naegeli and Cramer, and by 

 Bohm, but no explanation was offered of the connexion 

 between chlorophyll-corpuscle and starch-grain. It was left 

 to Sachs to do this. In endeavouring to determine fully the 

 conditions of the decomposition of carbon dioxide by green 

 plants, he had become impressed with the importance of 

 chlorophyll in the process. The fact that in the absence of 

 chlorophyll no decomposition of carbon dioxide takes place, 

 led him to investigate the structure of the chlorophyll-cor- 

 puscles. As the result of careful and extended observation 

 he found that the presence of starch-grains in chlorophyll- 

 corpuscles is dependent upon exposure to light. The for- 

 mation of starch-grains in chlorophyll-corpuscles was thus 

 shewn to depend upon the same conditions as the decompo- 

 sition of carbon dioxide, and Sachs was therefore justified 



