15-1: 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. 



llo. 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 at'ts as a solvent of the soil 

 materials and carries them into the plant. The water in 

 excess of the small amount used in fodd manufacture is 

 given ofE from the plant in the form of water vu.por, the 

 process being already referred to as transjiirdiiou (see §20). 



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 ahle to manufac- 

 tur(! much more food than they use, and it is upon this ox- 

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

 foods throuu'li 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 tlic^ plant, known as protoplasm, and the pi'otoplasm 

 builds the plant structure. This process of organizing the 

 food into the living substance is known as assimiliifion. 



11(1. Respiration. — The formation of foods, tlieir diges- 

 tion and assimilation are all preparatory to the process of 

 respiration, whi(di may be called the use of assimilated 

 food. The whole working power of tlu^ plant depends 



