DIFFUSION AND OSMOTIC PRESSURE. 229 



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 simul- 

 taneous occurrence is possible only because the roots are 

 neither truly semi-permeable membranes 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, &c.), the osmotic pressure, aided by 

 a rise in temperature, becomes greatly increased, and, as a con- 

 sequence, 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* and others 



* Proc. Roy. Soc. 62 t!897), 100. 



