144 VEaETABLE PHYSIOLOGY. 



nitrogen), of water [oxygen and hydrogen), and the sulphur of soluble sulphates, supply 

 most of the materials of vegetables. The carbon of carbonic acid by uniting with 

 the elements of water forms cellulose, sugar, gum, starch, &c. ; an excess of oxygen 

 produces vegetable acids (malic, citric, acetic, gallic, &c.) ; an excess of hydrogen 

 produces chlorophyll, oils, resins ; the azote of ammonia, added to the elements of 

 water and of carbonic acid, gives rise to vegetable alkalies [quinine, morphine, &c.) ; 

 finally, sulphur and phosphorus, combined with azote, oxygen, hydrogen, and 

 carbon, form three organic substances of similar composition, namely, jibrine, 

 albumen, and casern; these supply the animal kingdom with essentially nutritious 

 elements ; as stated above, they are always found in the blood, united with other 

 substances, and notably with a certain quantity of phosphate of lime, a salt which 

 constitutes the solid part of bones. 



Humus or mould is the name given to the black carbonaceous matter which 

 results from the decomposition of organic substances ; vegetable mould is nothing 

 but cellulose, which burns slowly under the influence of the oxygen of the atmosphere, 

 and changes into carbonic acid, which, dissolving in the water of the soil, passes into 

 the substance of the vegetable. The decomposition of the mould is assisted by 

 mineral alkalies [potash, soda, chalk, magnesia), which induce the formation of 

 carbonic acid, and form with it soluble carbonates, absorbed by the roots ; then, 

 under the influence of these same alkalies, the water and carbonic acid decompose, 

 and vegetable acids are formed, more or less oxygenized, with which they combine ; 

 finally, these acids change, and become sugar, starch, or cellulose. 



Thus, vegetable acids are indispensable to the existence of plants, and their 

 formation depends : 1, on the water and carbonic acid which combine to form them ; 

 2, on the mineral alkalies which induce this combination. Now these alkaline bases, 

 which play so important a part in vegetation, reside in hard or soft rocks, named 

 feldspar, mica, granite, gneiss, basalt, the elements of which are silica, alwmina, potash, 

 magnesia, lime, &c. ; these bases are liberated by the disintegration or decomposition 

 of the rocks, of which the debris, more or less changed, constitute arable soil. The 

 rocks are disintegrated by the water which, having penetrated them, expands in 

 passing to the state of ice, and thus overcomes the cohesion of their elements. 

 These elements are then dissolved by water, either pure, or containing oxygen, or 

 loaded with carbonic acid ; it is thus that the aluminous and alkaline silicates are 

 disintegrated and dissolved, previous to forming argillaceous soils. 



Alkalies, and especially potash, when mixed with soil, are rendered soluble by 

 the addition of sulphate of lime, as Deherain has proved. Since the sulphate of lime 

 changes the salts of potash into sulphate of potash, it has been supposed that the 

 greater solubility of potash after being thus treated is attributable to this trans- 

 formation; this hypothesis has not yet been practically proved, and we do not know 

 whether the sulphate acts chemically on the potash, or whether its effects are purely 

 physical, the object being to liquefy the soluble salts, to preserve them from the ab- 

 sorbent action of the earth, and to facilitate their absorption by the roots of the plant. 

 But, whatever be the explanation, this property of sulphate of lime proves the advantage 

 of adding it to the soil in which leguminous fodders are cultivated [Trefoil, Lucerne, 



