130 PLANT PHYSIOLOGY 



they cannot pass through the spaces between the molecules, 

 or groups of molecules, of other substances. On the other 

 hand, a colloidal membrane is no bar to the diffusion of 

 crystalloids, for though its molecules and groups of mole- 

 cules are large, it is supposed that the spaces between are 

 large enough for smaller molecules to pass through. Thus, 

 apart from Hertwig's conception ( p. 7) that the living proto- 

 plasm is a definite structure and not a substance or mixture 

 of substances merely, we have a reason why the protoplasm, 

 while permitting the free passage of water and of the sub- 

 stances dissolved in it, remains enclosed within the cell- wall, 

 although by the absorption of much water, or for other rea- 

 sons, the density of the protoplasm may be greatly reduced. 

 In this sense the living cell is an apparatus that permits 

 endosmosis while preventing exosmosis. In addition to the 

 failure of protoplasmic (i. e. of coUoidal) substances to 

 pass through the cell- wall because of the size of their mole- 

 cules, the living protoplasm (see p. 107) prevents by its 

 bounding membranes the exosmosis of dissolved coloring 

 and of some other substances contained in the vacuoles. 



SAP-PKESSURE AND BLEEDING 



The transfer of single cells containing osmotically active 

 substances in abundance — ^for example, ripe pollen-grains — 

 into pure water, or to an aqueous solution of too low den- 

 sity, will cause the cells to swell and finally to burst, in 

 consequence of turgor-pressure which finally ruptures the 

 cell-walls. Such cells would not swell and burst in air, or in 

 a solution nearly or quite equalling the cell-sap in density. 

 Similarly, the accumulation of osmotically active substances 

 in some of the cells of a multicellular plant which can ab- 

 sorb water in abundance, will result in developing pressure 

 — ^turgor-pressure — ^in those cells. These, exerting pressure 

 upon neighboring cells, will transmit the mechanical force, 

 often for a considerable distance, and it may ultimately be 

 exerted directly upon the soil or other material surrounding 

 the plant. The materials causing the development of pres- 

 sure may pass by osmosis to the cells against which pressure 

 is exerted. Thus, though the pressure in one cell or several 



