80 NUTRITION. 



The earthy acid called ulmin by Braconnot, and the 

 carbonaceous matter, are isomeric bodies. The quan- 

 tity of the extract, says Saussure, which boiling water 

 separates from mould, is not much, but the great nour- 

 ishing property of it is strikingly illustrated in the fact 

 that when seeds were grown in mould which had not 

 been boiled, the plants were one quarter greater than 

 they were when the mould had been boiled. 



44. At one time it was thought that nitrogen had little 

 or nothing to do in respect to the nourishment of 

 plants, and few cases only were supposed to exist in 

 which this element seemed an active ingredient. Gay 

 Lussac proved its presence in a very great number of 

 seeds, and lately Boussingsalt has followed him with 

 like results, and shown that Vetches, Lentils, and Beans, 

 have from 4 to 5 per cent, whilst Clover contains 7. 

 Besides this the presence of azote in the other organs of 

 vegetables is now demonstrated more frequently than 

 before; and Hermbstadt has shown that the cereal 

 grasses cultivated under the influence of the most fully 

 azotised soils, contain most gluten ; and it is highly 

 probable that the deterioration of a soil by crops grown 

 in it, depends on a great measure on the loss of its ni- 

 trogen abstracted from it by the plants. 



45. Although we must undoubtedly say with Thaer, 

 Boussingsalt, and others, that the manure or mould is 

 the agent contributing most efficaciously to the nourish- 

 ment of plants, and that the force of vegetation is de- 

 termined by the proportion of the nutritive fluids which 

 they there meet with, yet they likewise derive much 

 from the atmosphere and the objects contained in it, 

 and which find an entrance into the plant by means of 

 the leaves and foliaceous parts, &c. ; but this we shall 

 notice a little further on. 



46. Sap. The fluid along with its contained nutri- 

 tious materials, as soon as it enters the plant receives 

 the name of common or crude sap, but which is to be 

 distinguished from the true or proper sap, which is the 

 former properly digested in the leaves. 



