178 THE FOOD AND NUTRITION OF PLANTS. 



walls of the cells that form the surface, principally those of the 

 newest roots and their fibrils (133). The fluid absorbed by the 

 roots, mingled in the cells with some previously assimilated matter 

 they contain in solution (26, 79), is diffused by exosmosis and endos- 

 mosis from cell to cell, rising principally in the wood (224, 230) ; 

 and is attracted into the leaves (or to other parts of the surface of 

 the plant exposed to the air and light) by the exhalation which 

 takes place from them (314), and the consequent inspissation of the 

 sap. Here, exposed to the light of the sun, the crude sap is assimi- 

 lated, or converted into organizable matter (79) ; and, thus prepared 

 to form vegetable tissue or any organic product, the elaborated fluid 

 is attracted into growing parts by endosmosis, in consequence of its 

 consumption and condensation there, or is diffused through the newer 

 tissues. There is no movement in plants of the nature of the cir- 

 culation in animals. Even in the so-called vessels of the latex 

 there is merely a mechanical flow from the turgid tubes towards the 

 place where the liquid is escaping when wounded, or from a part 

 placed under increased pressure (63). The only circulation, or 

 directly vital movement of fluid, in vegetable tissue, is the cyclosis, 

 or the system of currents in the layer of protoplasm in young and 

 active cells (36) : this movement is confined to the individual cell, 

 and can have no influence in the transference of the sap from cell 

 to cell. Respiration is likewise a function of animals alone. "Wliat 

 is generally so called in vegetables is connected with assimilation, 

 and is of entirely different physiological significance, as will pres- 

 ently be shoAvn. None of the secretions of plants appear, like 

 many of those of animals, to play any part, at least any essential 

 part, in nutrition. Many, if not all of them, are purely chemical 

 transformations of the general assimilated products of plants, — are 

 excretions rather than secretions (88 — 90). 



320. The appropriation of assimilated matter in vegetable growth, 

 and the production and multiplication of cells, Avhich make up the 

 fabric of the plant, have already been treated of (25-34). We 

 have now mainly to consider what the food of plants is, whence it is 

 derived, and how it is elaborated. 



