CHAPTER XIII 



PHYSIOLOGY 

 Nutrition, Respiration, Growth, and Irritability 



THE chemical elements necessary for the normal growth of plants 

 have already been referred to (Chap. I), and we have now to con- 

 sider how these are taken in and used by the plant. In ordinary 

 plant-tissues, the presence of a cell-wall precludes the entrance into 

 the cells of solid particles. Where the protoplast is not enclosed 

 by a cell-wall, as in the plasmodia of the Slime-moulds, solid bodies 

 are ingested, and, within the cells of ordinary plants, solid bodies, 

 like starch-granules or crystals, may be taken up from the vacuoles 

 by the protoplast, or ejected into the cell-sap. It is necessary, how- 

 ever, before such bodies can be incorporated into the substance of 

 the protoplast, that they should be dissolved, and all the food of the 

 plant, before it can be used, must be in soluble form. 



FOOD OF PLANTS 



All food substances enter the plant in the form of compounds of 

 greater or less complexity. These undergo many complicated changes, 

 destructive and constructive, before they are incorporated into the 

 living substance of the protoplasm. It is these changes which com- 

 prise the nutritive processes of the plant, the destruction of certain 

 substances being necessary to furnish the energy as well as the 

 chemical constituents required for the constructive activity. Fuel 

 as a source of energy is as necessary for the living engine as it is 

 for the mechanical one. Two principal types of products result from 

 these activities : (1) plastic substances, or those which are used to 

 build up the tissues and are capable of various transformations, like 

 starch, and various proteids ; (2) aplastic substances, which, once 

 formed, are incapable of further transformations i.e. crystals of 

 calcium oxalate, wood, resin, cork, and the other dead substances of 

 the plant. Another important group of compounds, which do not 

 themselves take part in building up protoplasm, are the enzymes, 



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