173 FOUNDATIONS OF BOTANY 
pound (lignin) characteristic of wood-fiber, that go to make 
up the main bulk of a large tree must be carried on in the 
roots, trunk, and branches of the tree. 
184. Digestive Metabolism. — Plant-food in order to be 
carried to the parts where it is needed must be dissolved, 
and this dissolving often involves a chemical change and 
is somewhat similar to digestion as it occurs in animals. 
The newly made starch in the leaf must be changed to a 
sugar or other substance soluble in water before it can be 
carried to the parts of the plant where it is to be stored 
or to rapidly growing parts where it is to be used for 
building material. On the other hand, starch, oil, and 
such insoluble proteids as are deposited in the outer por- 
tion of the kernel of wheat and other grains are extremely 
well adapted to serve as stored food, but on account of 
their insoluble nature are quite unfit to circulate through 
the tissues of the plant. The various kinds of sugar are 
not well adapted for storage, since they ferment easily in 
the presence of warmth and moisture if yeast-cells or 
suitable kinds of bacteria are present. 
Two important differences between starch-making in 
the green parts of plants and the non-constructive or the 
destructive type of metabolism should be carefully noticed. 
These latter kinds of metabolism go on in the dark as 
well as in the light and do not add to the total weight 
of the plant. 
185. Excretion of Water and Respiration. — Enough has 
been said in Sect. 174 concerning the former of these pro- 
cesses. Respiration, or breathing in oxygen and giving 
off carbonic acid gas, is an operation which goes on con- 
stantly in plants, as it does in animals, and is necessary to 
