28 A TEXTBOOK OF BOTANY [Ch. Ill, 4 



used in forming grape sugar, does not become obliterated in 

 the process, but is simply converted into the latent or po- 

 tential form. Thus the grape sugar becomes a store of 

 potential energy, which is retained through the later trans- 

 formations, and which can be released and rendered again 

 active by the process of respiration, as we shall later describe 

 in full. Grape sugar, accordingly, and its derivatives are the 

 source both of the materials and the energy used by plants 

 in their growth and work, or, in other words, are their food. 

 Furthermore, since all animals are dependent upon plants, 

 either directly or indirectly, for their food, the photosynthetic 

 grape sugar is the basal food for all animals also. 



This use of the term plant food may seem strange to those 

 who know the common application of the word to the min- 

 eral salts taken by plants from the soil. The latter usage, 

 though well sanctioned by custom, especially in connection 

 with agriculture, is physiologically erroneous. Food, in the 

 physiology of both animals and plants, is that material from 

 which the living body is constructed, and energy obtained 

 for its work. It is because the mineral salts of the soil supply 

 only an insignificant fraction of the substance of plants and 

 none at all of their energy that they cannot be considered 

 plant food, while the name belongs properly to grape sugar, 

 which supplies both. The popular usage arose before these 

 matters were understood, but is too firmly fixed to be changed. 

 No confusion can arise if one takes note of the connection 

 in which the word is employed. 



4. The Cellular Anatomy of Leaves 



The actual process of photosynthetic food-formation is 

 performed in the cells of the leaf, to which we now turn at- 

 tention. For this study we use the compound microscope, 

 which is the indispensable tool of the biologist, and one of 

 the most powerful and perfect of all the exact instruments 

 which scientific men have invented to extend the range and 

 precision of our limited senses. 



