MINUTE STRUCTURE OF LEAVES. 1238 
known by the general name of metabolism. The change of 
starch into grape sugar or maltose is a characteristic instance 
of the non-constructive kind of metabolism. It is essential to 
the growth of the plant that many and complicated transfor- 
mations of material should occur within it; for example, 
starch, oil, and such insoluble proteids as are deposited in the 
outer portion of the kernel of wheat and other grains are’ 
extremely well adapted to serve as stored nutriment, but, on 
account of their insoluble nature, are quite unfit to circulate 
through the tissues of the plant. The various kinds of sugar, 
on the other hand, are not well adapted for storage, since they 
ferment easily in the presence of warmth and moisture. 
By metabolic processes the tissues of the plant and their 
contents are all constructed out of certain formative materials. 
From starch, sugars, or fats, cellulose, the material of ordinary 
cell walls, is made, and from various proteids protoplasm and 
the chlorophyll bodies are produced. 
Two important differénces between fixation of carbon and 
the non-constructive or destructive type of metabolism should 
be carefully noticed. . Destructive metabolism goes on in the 
dark as well as in the light, and it does not add to the total 
weight of the plant. 
157. Excretion of Water and Respiration. — Enough has 
been said in § 145 concerning the former of these processes. 
Respiration or breathing in oxygen and giving off carbonic acid 
gas is an operation which goes on constantly in plants, as it does 
in animals, and is necessary to their life. For, like animals, 
plants get the energy with which they do the work of assimila- 
tion, growth, reproduction, and performing their movements, 
from the oxidation or burning up of such combustible substances 
as they can use for that purpose ; for instance, starch and sugar.’ 
1The necessity of an air supply about the roots of the plant may be shown by 
filling the pot or jar in which the hydrangea was grown, for the transpiration exper- 
iment, perfectly full of water and noting the subsequent appearance of the plant at 
periods 12 to 24 hours apart. 
