564 PHYSIOLOGY. 



salts are absorbed in different proportions from mixed solutions ; and in 

 Be Saussure's experiments living roots absorbed differently from diseased 

 or dead ones. Similar inferences may be drawn from the effects of 

 manures : thus Cereals are specially benefited by nitrogenous manures, 

 Leguminous plants by mineral manures ; and yet the ash-analysis of the 

 former shows that they contain a less percentage of nitrogenous matters 

 than do the Leguminosae. Wheat crops and grasses generally, according 

 to the experiments of Lawes and Gilbert, supply themselves with diffi- 

 culty with nitrogen, while Clover and other Leguminosee take it up 

 freely. 



Such phenomena as these, however, may be explicable on purely phy- 

 sical principles. It has been proved that different chemical salts exhibit 

 unlike quantitative phenomena in passing through dead endosmotic sub- 

 stances ; and thus even from mixed fluids one salt might pass more readily 

 into a cell than another j and, still more, the immediate decomposition of 

 one salt alone, inside the membrane, while the other was not affected, 

 which might take place in a living cell, would greatly affect the endos- 

 mose, since the cell-contents would soon be saturated with the latter, 

 while the other would not accumulate. According to Knop, the roots 

 will absorb from solutions of nutritive salts an amount proportionate to 

 the degree of concentration of the solution : thus the stronger the solu- 

 tion the more dilute the liquid absorbed by the root ; on the other hand, 

 if the solution be less concentrated, the root will take up a relatively larger 

 quantity of water than of the salt. In regard to De Saussure's experi- 

 ments (which are borne out by what we see beneath the microscope when 

 we apply reagents, such as iodine, to healthy or decaying tissues), there is 

 no necessity to have recourse to a vital agency of' selection, since the 

 chemical activity of the cell-contents, quite different in a living and in a 

 dead organism, might account for all the diversities, even if the difference 

 could not be explained by a physical difference of tension in the living 

 cell-membrane and that of a dead organ, in which a process of decay im- 

 mediately commences if it is exposed to the action of water. 



It has recently been shown that porous vessels placed in mixed solu- 

 tions select, just as plants do under similar circumstances; and those 

 solutions which pass most freely through the walls of cells are those which 

 always pass most freely through the sides of the porous vessels. Those 

 cases' in which the same amount of any given substance is capable of being 

 absorbed by plants which have nevertheless different chemical composi- 

 tion, may also be explained by the different osmotic powers possessed by 

 the cells of different plants. Thus, supposing the root-cells of a Cereal 

 plant and those of a Leguminous plant to take up the same amount of 

 silica from the soil, the quantity of that ingredient would speedily be 

 found to be greater in the Cereal than in the Leguminous plant, because 

 the cells of the former can appropriate silica, and by osmosis store it up 

 in the epidermal tissues, while the cells of the latter, having different 

 osmotic relations to silica, soon become saturated and can take up no 

 more. On the same principle we see cells in juxtaposition containing 

 very different ingredients, which yet do not mix because the conditions 

 for'endosmosis are in some way or other not favourable. 



Influence of Evaporation. Schlosing says that the power of absorbing 

 mineral ingredients from the soil is diminished by limiting the process 



