154 PLANT RELATIONS. 



hydrates, the nitrogen, sulphur, and other necessary 

 additional elements being obtained from soil substances 

 dissolved in the water which is absorbed and conveyed 

 to the leaves. 



113. Transpiration. — The water which is absorbed by the 

 roots and passes to the leaves is much more abundant than 

 is needed in the process of photosynthesis. It should be re- 

 membered that the water is not only used as a raw material 

 for food manufacture, but also acts as a solvent of the soil 

 materials that are passing into the plant. The water in 

 excess of the small amount used in food manufacture is 

 given off from the plant in the form of water vapor, the 

 process being already referred to as transpiration (see §26). 



114. Digestion. — Carbohydrates and proteids may be re- 

 garded as prominent types of plant food which green 

 plants are able to manufacture. These foods are trans- 

 ported through the plant to regions where work is going on, 

 and if there is a greater supply of food than is needed for 

 the working regions, the excess is stored up in some part 

 of the plant. As a rule, green plants are able to manufac- 

 ture much more food than they use, and it is upon this ex- 

 cess that other plants and animals live. In the transfer of 

 foods through the plant certain changes are often neces- 

 sary. For example, starch is insoluble, and hence cannot 

 be carried about in solution. It is necessary to transform 

 it into sugar, which is soluble. These changes, made to 

 facilitate the transfer of foods, represent digestion. 



115. Assimilation. — When food in some form has reached 

 a working region, it is organized into the living substance 

 of the plant, known as protoplasm, and the protoplasm 

 builds the plant structure. This process of organizing the 

 food into the living substance is known as assimilation. 



11G. Respiration. — The formation of foods, their diges- 

 tion and assimilation are all preparatory to the process of 

 respiration, which may be called the use of assimilated 

 food. The whole working power of the plant depends 



