122 ELEMENTS OF BOTANY. 
155. Assimilation.— The fixation of carbonic acid, by 
combining a part of its constituents with a part of the con- 
stituents of water, to form starch, is only one special, though 
very important, case of assimilation, that is, of the manu- 
facture by the plant, from foreign materials, of the chemical 
compounds which make up its substance. <A rather better 
term than assimilation is constructive metabolism. Besides 
carbonic acid gas and water, ordinary green plants require 
as food some compound of nitrogen, such as nitrates and 
ammonium compounds, sulphur and phosphorus, in suitable 
combinations, compounds of iron, calcium, potassium, and, 
perhaps, of sodium and of chlorine.} 
These substances are found occurring in minute quantities 
in the soil-water and in ordinary flowering plants are brought 
to the parenchyma cells of the leaves or of the green layer of 
the bark to be worked over into the constituents of the plant. 
All parts of the process are due to the activity of the proto- 
plasm contained in the cells of the working portions of the 
plant. Protoplasm is the jelly-like or semi-fluid proteid sub- 
stance to which the life and working power of every active 
cell are due. The student has already become acquainted 
with protoplasm, since most of the tissues which he has 
examined, except the epidermis, the dead portions of the 
corky layer of the bark, the heartwood, and the dry pith, 
have been composed of cells which contained much proto- 
plasm and some of which, as the cambium layer, contained 
little else but protoplasm. 
156. Non-Constructive Metabolism.?— Side by side with 
the transformation of the inorganic substances drawn from 
earth and air into starch, protoplasm and other characteristic 
vegetable substances, there occur a series of other changes 
1 There is evidently room for the teacher, if he wishes, to do much in the way of 
exhibiting to the class the chemical compounds from which, as raw materials, plants 
manufacture their tissues. 
2 See Werner and Oliver’s Natural History of Plants, vol. I, pp. 455-465. 
