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know not only the definite chemical elements which are essential 

 for plant life, but we know also the quantity and form in which 

 they are most favorable for plant growth. Having established 

 this, it is possible to understand the role of plants in the general 

 economy of the world, and how their manner of life, in a broad 

 sense, supplements that of animals. There is also pretty 

 definite information as to the physical phenomena connected with 

 the absorption of the raw food materials which the plant after- 

 wards 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 

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

 solutions of equal concentration ; why it cannot 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 substances 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 carbo- 

 hydrate 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 



