CHAPTER VI 



THE INCOME, MOVEMENTS, AND OUTGO 

 OF MATERIALS IN PLANTS 



Cells are Bags of Water. — Cells, and organisms 

 themselves, are essentially bags of water. The physico- 

 chemical systems, cells composed of water and colloids, 

 arranging themselves into self-operating mechanisms, are 

 either sac-like, the contents of the sacs or bags being 

 aqueous solutions enclosed in walls of living protoplasm, 

 or the protoplasm is disposed in a sponge-like mass en- 

 closing drops of water. In either case, the behavior of 

 living cells is that of bags of water, so far as income, 

 movements, and outgo of materials are concerned. In 

 plants the sacs or bags are mechanically supported and 

 strengthened by enclosing cell walls of cellulose. Dis- 

 solved in the cell sap are many substances, so finely 

 divided that molecular movements and chemical changes 

 may take place which would be impossible in the solid 

 state. Thus we find diffusion of a dissolved substance 

 taking place throughout a volume of liquid and a per- 

 fectly uniform distribution resulting; but the movement 

 of dissolved substances into and out of cells implies the 

 ability of the molecules of the dissolved substance to 

 pass through such membranes as the colloidal system 

 which we have described possesses. 



Permeability. — On the surface of contact between 

 two different materials, as for example between proto- 

 plasm and cell wall, between nucleus and cytoplasm, an 

 actual membrane is formed by the living cytoplasm. 

 This membrane may be too thin to be seen under the 

 microscope, but its existence may be otherwise proved. 

 Thus the coloring matter in roots and other parts of red 



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