the absorption of the raw food materials which the 

 plant afterwards elaborates, information which is largely 

 due to the classic researches of Pfeffer, whose work, it 

 may be remarked, also afforded Van t' Hoff valuable 

 data for his contributions to the establishment of the 

 modern physical chemistry. Application of the laws of 

 diffusion and of osmosis, as shown by Pfeffer, enables us 

 to understand why a plant may absorb more of one min- 

 eral salt than of another, though both be presented to it in 

 solutions of equal concentration; why it cannit absorb 

 some substances at all, while on the other hand it cannot 

 avoid absorbing certain substances, even though they be 

 violent poison and kill the protoplasm of the absorbing 

 cell at once. We understand also a good deal of the 

 mechanism of the production from simple inorganic sub- 

 stances of the first organic food by the green plant, the 

 first organic food of the whole organic world. While, as 

 will be shown later, the precise details of this process are 

 not fully understood, the general facts are a matter of 

 almost common information, so well known that I hesitate 

 to speak of it here, though to sum up the matter in a few 

 words it may be said that this process of photosynthetic 

 activity of green plants is carried on by the living cells in 

 the presence of sunlight, through the agency of the green 

 coloring matter chlorophyll which is present in the 

 leaves, and that the chemical reaction involved results in 

 the union of the carbon dioxide absorbed from the. air, 

 with water absorbed from the soil, to form the first simple 

 carbohydrate that is to be detected in easily recognizable 

 form as starch. The fact that this process takes place does 

 not interfere with the operation of another one, namely the 

 absorption of oxygen with the giving forth of carbon 

 dioxide, that is concerned in the mechanism of respiration. 

 Respiration as a means of releasing the stored energy in 

 available form for the constructive work of the organism 



17 



