viii.] TRANSFER OF FLUIDS. 105 



fluids must be of different chemical composition, or of 

 different 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 

 varying density, and the walls of the cells are permeable. 

 The consequence is, that there may be constant transmis- 

 sion 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 con- 

 sisting 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 re- 

 placed by air, it is possible that the transference of fluid 

 may be due to the existence of some molecular relation 

 established between the substance of the cellulose thick- 

 ening of the cell-walls and the particles of water by which 

 it is freely permeable, the precise nature of which we are 

 not yet in a position to define, but which may be currently 

 described as intermolecular. Whatever explanation may 

 be ultimately adopted, the fact is clear that the chief 

 upward transference of fluid takes place through the wood- 

 cells of the vascular bundles, which collectively form in 

 Dicotyledons the sap-wood or alburnum, and that this 

 transference during active spring-vegetation is very rapid. 

 In those woody dicotyledonous stems in which a distinct 

 duramen occurs, this central portion, after acquiring its 

 characteristic colour and other physical features, may be 

 regarded as relatively dead matter, taking no direct part 

 in the vital functions of the plant. The direction of the 

 current is mainly determined by the constant evapora- 

 tion from the leaves, so that the water taken up by the 

 surface-cells and hairs of the root-<fibrils is impelled up- 

 wards through the stem and branches, to restore the 



