NUTRITION. THE MACHINERY. 27 



becomes starch, and is stored up for future use in that form. 

 No starch is formed in an atmosphere purposely deprived 

 of carbonic acid by the experimenter, even if the cell be ex- 

 posed to the light. Moreover, any starch that may have 

 been previously formed disappears under such circum- 

 stances, just as it would do in darkness, where the plant is 

 dependent on its reserve stores for its nourishment, and 

 not on those which it procures directly for itself when ex- 

 posed to light in an atmosphere in which carbonic acid gas 

 forms a part. The changes in question are presumed to 

 take place, not in the protoplasm itself, but in the chloro- 

 phyll grains ; at any rate, it is in them that the starch first 

 makes its appearance. It is certain, also, that only cells 

 which contain chlorophyll and then only when exposed to 

 light can directly assimilate carbon. Cells without chloro- 

 phyll, such as those of fungi, obtain their carbon by more 

 indirect and complex means. The vital importance of the 

 exposure of the leaves to sunlight might be inferred from 

 the bending of the stems and branches to the light, and 

 the placing of the mobile leaves at such an angle as to 

 receive the full benefit of the sun's rays matters which 

 will be spoken of further on. 



What the Leaves do in Darkness. Inhalation of 

 Oxygen. In darkness (as well as under the influence of 

 light, in the case of those cells that do not contain chloro- 

 phyll) changes go on of a different character to those just 

 described. There is, in fact, a constant elimination of car- 

 bonic acid gas, and a corresponding absorption and retention 

 of oxygen gas. The interchange of these gases has been 

 compared to the corresponding changes in the case of the 

 respiration of animals ; but doubts have been thrown on 



