JSrUTEITION or VEGETABLES. 145 



Sainfoin), of whticli the ashes are rich in potash ; while, on the contrary, the addition 

 of carbonate of lime, which induces the formation of ammonia, is very usefully 

 employed in the cultivation of cereals, for which azotized manures are necessary. 



Silica is useful, because, being powdery and insoluble, it admits air and mois- 

 ture, alumina, because it retains moisture, around the roots ; lime, because, under the 

 influence of water acidified by carbonic acid, it replaces the alkaline bases of the 

 silicates ; hence the importance of marl, which is a mixture of clay and lime. If the 

 soil is composed of pure silica or of pure chalk, it is absolutely sterile ; if it is wholly 

 of c^ay, the roots cannot penetrate it. The best soil is that in which clay is mixed 

 with carbonate of lime and sand {silica), in such proportions that air and moisture 

 readily permeate it. 



Tillage improves the soil by breaking it up, and multiplying the surfaces which 

 ought to be in contact with carbonic acid, the ammonia of the rain, and the oxygen 

 of the air, so that the debris of the rocks may be rendered soluble, and form arable 

 land. The period of fallow is that during which the soil is left to atmospheric 

 influences. While the land is thus left fallow as a preparation for certain crops, it 

 may be occupied by some other plant which does not rob the soil of the materials 

 required for such crops ; this explains the theory of the rotation of crops. 



NUTRITION OF VEGETABLES. 



Absorption. — The roots are the principal organs of absorption ; they pump up the 

 liquid into which they are plunged, by means of their permeable cells. The upward 

 movement of the sap is explained by a recent discovery in physics : — if a tube closed 

 below by a porous membrane, and filled with a dense liquid, is plunged into a less 

 dense coloured liquid, there is soon a tendency to establish an equilibrium of density, 

 and the dense liquid in the tube becomes coloured by the addition of the less dense 

 liquid outside it, and the two liquids stand at different heights ; that in the plunged 

 tube rises above its level, and only stops rising when its density is no longer greater 

 than that in the outer tube. But to produce this equilibrium, the exterior liquid 

 must receive a certain quantity of that within ; thus there is a double current esta- 

 blished through the porous membrane ; the one from without inwards, called endos- 

 mose ; the other, less in degree, from within outwards, called exosmose. . This action 

 accompanies the absorption of fluid by the roots ; the damp soil contains water laden 

 with ammonia, carbonic acid, and different salts ; the roots, as well as the stem, are 

 composed of a series of superimposed cells, some of which are filled with a dense juice, 

 and others with vessels in which the liquid can easily rise by capillary action ; the 

 spongioles which terminate the root- fibres having no epidermis, are very permeable, 

 the water of the soil penetrates them, the juice which they contain is diluted by 

 this water, and to establish equilibrium the sap rises from cell to cell to the top of 

 the plant. 



Circulation. — When the water of the soil, laden with the carbonic acid, ammonia, 

 and mineral matters dissolved in it, has peneti-ated the plant, it takes the name of 



li 



