122 ELEMENTS OP 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 



' 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. 



' See Eorner and Oliver's Natural Bistory of Plants, vol. I, pp. 455-465. 



