138 THE LIFE OF THE PLANT 



in the dark. Thus starch is formed and carbonic acid 

 decomposed only in presence of chlorophyll ; both 

 processes are conditioned by the presence of light, and 

 only when carbonic acid is present and being decomposed 

 does any starch appear. 



It therefore seems more and more evident that starch 

 is the very substance we are looking for, which is 

 formed out of the carbon in carbonic acid. Its com- 

 position supports the conclusion : like other carbo- 

 hydrates it can be regarded, as the name suggests, as 

 made up of carbon and water. Cells always contain 

 water ; so we may explain the origin of a carbohydrate 

 by supposing water and carbonic acid to combine, 

 and at the same time all the oxygen of the latter to be 

 withdrawn. Such is the course of events as far as they 

 are known to us ; but we must remember that our in- 

 formation on this subject is as yet far from being 

 complete. We know that a cell receives carbonic acid 

 and water, gives off oxygen and forms a carbohydrate ; 

 we know that these processes must have a causal 

 connection, that they take place in the same chloro- 

 plast and follow upon each other with striking 

 rapidity. As to how it all happens, where the oxygen 

 comes from, whether it is produced entirely from 

 carbonic acid or also partly from water (which is more 

 probable), and whether other simpler or perhaps more 

 complicated combinations precede the formation of 

 starch up to the present we do not know anything 

 about these questions, and it would certainly be out of 

 place to enter here into speculative comment upon 

 phenomena as yet unexplained by science. 



It is far more important to remember that, in observ- 

 ing these processes of carbonic acid decomposition and 

 starch formation, we are witnessing one of the most 

 important phenomena of life, one on which depends not 

 only the life of the leaf and of the plant, but the life of 

 the whole organic world. This transformation of the 



