ifO DIFFUSION. (CHA1». 



admitted by healthy uninjured plants. Mistakes as to this 

 point have arisen from using injured or wounded plants for 

 experiment. Therefore only fluids, substances dissolved in 

 fluids, and gases, can be absorbed by the plant ; viz. fluids 

 with solids or gases in solution by the roots, gases and 

 vapour by the leaves. 



The circulation or transference of these fluids and gases 

 from cell to cell can only be by diffusion^ a physical process, 

 probably controlled, in some way not yet imitable, by its 

 taking place in a living apparatus. This process of diff'usion 

 depends upon two conditions. First, we must have two 

 fluids separated by a membrane of some kind which they 

 can permeate. Second, these fluids must be of different 

 chemical composition, or of difl'erent density. When these 

 conditions exist, a current is set up through the membrane, 

 which results in one of the fluids (the denser) increasing in 

 bulk at the expense of the other. This increase is due to 

 Diffusion. The affinity of the membrane itself for one of the 

 fluids in preference to the other modifies the result. Now 

 these conditions obtain throughout all plants, excepting, of 

 course, the old dead and dry portions of trunks, &c. They 

 are built up of closed cells, containing fluids of various 

 density, and the walls of the cells are permeable. The con- 

 sequence is, that there is a constant transmission of fluids 

 going forward throughout their tissues. This explanation of 

 the transference of fluids in plants no doubt is satisfactory so 

 far as it applies to tissues consisting of cells with relatively 

 large cavities, but when we find, as in many woody stems, 

 that the course of the fluid, the so-called " ascending sap," 

 absorbed by the root, is chiefly through the thick-walled 

 prosenchyma of the wood, the individual cells of which have 

 their walls much thickened, and their protoplasmic contents 

 mainly replaced by air, we are compelled to attribute the 



