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1 2/ which thus absorbs in its turn, and so on for cells ' 3 ' and '4/ 

 until a state of equilibrium being for the moment established 

 between cells ' 3 ' and ' 4,' the fluid makes its way to the central 

 core. By this time the epidermal cell, having its cell-sap 

 rendered stronger through the removal of its absorbed water by 

 cell ' 1/ takes more from the soil, which in like manner is 

 passed on as before. Hence water is being continually absorbed 

 from the soil by the root-hairs, and passed on from cell to cell 

 through the parenchyma, by a process of endosmosis. But 

 when we come to the passage of the fluid from the thin-walled 

 parenchyma tissue to the cells of the thick-walled woody core, 

 a difficulty arises, since the cells of the latter contain air, and it 

 is not possible that this passage can take place by an ordinary pro- 

 cess of diffusion, for in the case before us we have air on one side 

 of the cell -wall, and a liquid on the other, whereas diffusion re- 

 quires the presence of two liquids, one on either side of the 

 separating membrane. The explanation is, that as the fluid is 

 taken up by the parenchyma cells from without, through the 

 medium of the root-hairs, these cells become turgid, and in 

 virtue of this, by the osmotic action of their cell-sap they take up 

 more water than they can possibly contain — or, in other words, 

 the osmotic activity of the cells is so great that it is in excess of 

 their cubic content; they become in consequence gorged with 

 water, and in this manner a hydrostatic pressure or tension is set 

 up, and filtration takes place into the cells of the central core. 

 Filtration differs from diffusion in that it can take place where 

 there is fluid on one side only of the separating membrane, and air 

 on the other. We can show that water exists in the cells or 

 vessels of the central core, by putting a damp log on the fire 

 when we find that fluid and air bubbles come out. The vessels 

 are simply formed by rows of cells placed vertically one above 

 another, the partition walls of which have become absorbed; 

 afterwards the cell-walls of these vessels so formed undergo 

 chemical change. Next we have to enquire what becomes of 

 the water when it has found its way into the vessels in the way 

 described : in order to do this we must understand the structure 

 of leaves, and something also concerning transpiration. 



