OSMOSIS AND ABSORPTION BY ROOTS. 65 



other properties in common, that without the aid of 

 chemical means we cannot accurately distinguish the one 

 from the other. But the plant clearly discriminates 

 between the two salts, for it separates the one from the 

 other, and for every one equivalent of potassium which 

 it absorbs leaves behind in the water more than thirty 

 equivalents of sodium. Manganese and iron, iodine 

 and chlorine, are likewise isomorphous bodies ; yet the 

 iodine plant separates one equivalent of iodine in sea- 

 water from many thousand equivalents of chlorine. 



The known laws of osmosis, and of the diffusion or 

 interchange of water and salts through a dead mem- 

 brane or a porous mineral body, give no explanation 

 whatever of the action exercised by a living membrane 

 upon salts in solution, or how they pass through it into 

 the plant. The observations of Graham (' Phil. Mag.' 

 ser. IV. August 1850) show that matters capable of 

 exerting a chemical action upon animal membranes, 

 such as carbonate of potash and caustic potash, causing 

 them to swell and gradually decomposing them, facili- 

 tate the passage of water to an extraordinary degree.* 

 Graham remarks that the processes of alteration, decom- 

 position, and new formation, which are incessantly 

 taking place in the membranes and cells in all parts of 

 the plant, and which we have no means of denning or 

 measuring, must entirely change the osmotic process : 

 the permeation of mineral substances through the living 

 vegetable membrane must,. therefore, be governed by 

 very complex laws. 



Land plants act in the same manner with respect to 

 the soil in which they grow, as marine plants to sea- 



* The water in the tubes of his osmometer rose to 167 millimeters, 

 when holding 1/1 Om. per cent, of carbonate of potash in solution; with 1 

 per cent, of that salt, it rose to 863 millimeters (38 inches, English). In 

 another experiment, the water holding 1 per cent, of sulphate of potash in 

 solution, rose to twelve millimeters ; upon the addition of 1/10 percent, of 

 carbonate of potash to the solution, it rose to 254-264 millimeters ; .the 

 same potash solution by itself rose only to 92 millimeters. The notion of 

 an osmotic equivalent is altogether inadmissible, if the membrane is 

 chemically altered. Graham's latest investigations on the dialysis of- 

 crystalline and amorphous bodies are extremely interesting, and promise to 

 throw considerable light upon the processes in the animal organism. 



