154 PLANT STUDIES 
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 and carries them 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. 
116. 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 
