226 OSMOSIS AND DIFFUSION 



A living vegetable cell has been employed as a means of detecting 

 the equality of the osmotic pressures existing in two solutions. If the 

 cell be surrounded with a solution in which the number of molecules 

 of dissolved substance per unit volume be greater than that in the sap 

 within it, the water will pass through the protoplasm out of the cell, 

 and the protoplasm will shrink from the rigid cellulose wall. This 

 phenomenon is known as plasmolysis and can be observed under the 

 microscope. With a certain strength of liquid the cell contents will 

 be in equilibrium, i.e., no water will leave or enter. Solutions of dif- 

 ferent salts have the same osmotic pressure or are isotonic if they are 

 in equilibrium with the sap of the same cell. 



As already stated, perfect semi-permeable membranes are not 

 known, and all membranes that have been observed permit of a slight 

 amount of diffusion of the dissolved matter as well as of the solvent 

 through them. 



It is therefore probable that diffusion of dissolved substances from 

 without and the setting up of osmotic pressure within the roots are 

 processes opposed to each other, and their simultaneous occurrence is 

 possible only because the roots are neither truly semi-permeable mem- 

 branes on the one hand, nor merely porous colloidal bodies, permitting 

 of free diffusion, on the other. 



The living protoplasm of a vegetable cell thus permits of slight 

 diffusion of its contents outwards and of the dissolved matters present 

 in the surrounding liquid, inwards, though at the same time showing 

 the production of internal osmotic pressure owing to the fact that the 

 liquid within itself is more concentrated in solid matter (i.e., contains 

 more molecules of dissolved substances per unit volume) than the 

 liquid without. In the spring, when the plant juices become richer in 

 dissolved matter (probably owing to the activity of ferments contained 

 in the tissues leading to the production of sugar from starch, amides 

 from albuminoids, etc.), the osmotic pressure, aided by a rise in tem- 

 perature, becomes greatly increased, and, as a consequence, the roots 

 of the plant, taking in large quantities of water from the soil, while 

 losing comparatively little by diffusion, set up root pressure, which 

 forces the sap up into the stem and leaves. 



The magnitude of this root pressure in certain plants has been 

 measured and found to rise sometimes to three or four atmospheres. 



It is through the roots, by diffusion, that the mineral matters and 

 the nitrogen (in the form of nitrates) required by a plant are taken in 

 and forced, largely by osmotic pressure (due, mainly, not to them, but 

 to the organic constituents present in the sap), up into the stem and 

 leaves. 



The diffusion of substances in solution from one part of the plant 

 to another is made easier and does not tend to set up osmotic pressure 

 between different parts of the plant, because of the fact revealed by 

 the researches of Gardiner l and others that the protoplasm is con- 

 tinuous from cell to cell, the continuity being maintained by fine 

 threads passing through minute perforations in the cell walls. Through 



1 Proc. Boy. Soo., 62 (1897), 100. 



