182 THE FOOD AND NUTRITION OF PLANTS. 



degree of simplicity in vegetables. Imbibition, assimilation, 

 growth, and perhaps secretion, apparently include the whole. 



319. Plants absorb their food, entirely in a liquid or gaseous 

 form, by imbibition, according to the law of endosmosis (37), 

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

 those of the newest roots and their fibrils (120). The fluid ab- 

 sorbed by the roots, mingled in the cells with some previously as- 

 similated matter they contain in solution (27, 79), is diffused by 

 exosmosis and endosmosis from cell to cell, aided by the capillary 

 action of the fibro-vascular tissue of the wood, through the newer 

 parts of which the sap principally rises in stems of some age (210, 

 217) ; and is attracted into the leaves (or to other parts of the sur- 

 face of the plant exposed to the air and light) by the exhalation 

 which takes place from them (314), and the consequent inspissa- 

 tion of the sap. Here, exposed to the light of the sun, the crude 

 sap is assimilated, or converted into organizable matter (79), with 

 the evolution of oxygen gas into the air ; and, thus prepared to form 

 vegetable tissue or any organic product, the elaborated fluid is at- 

 tracted into growing parts by endosmosis, in consequence of its con- 

 sumption and condensation there, or is diffused through the newer 

 tissues. The fluids are transferred from place to place by permea- 

 tion and diffusion, according to a simple physical law. There is 

 no movement in plants of the nature of the circulation in animals 

 (37). 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 in- 

 creased pressure (63). The only circulation, or directly vital 

 movement of fluid, in vegetable tissue, is that of rotation, or the sys- 

 tem of currents in or next 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. 

 What is so called in vegetables is connected with assimilation, 

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

 ently be shown. 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 (80). 



320. The appropriation of assimilated matter in vegetable growth, 



