262 MANUAL OF AGRICULTUKE. 



The same is true of the nitrogen which the plant contains. This 

 elementary body, in the form of albumen, or some other of the 

 numerous protein compounds which occur in the sap, presides 

 over, or takes part in, almost every important transformation 

 which the organic matter of the living vegetable undergoes. Thus 

 it is always abundantly present when the starch of the seed or of 

 the tuber is dissolved and sent up to feed the young shoot ; and 

 again, when the soluble substances of the sap are converted into 

 the starch of the grain of the tuber, or of the body or pith of the 

 tree, one or other of the protein combinations is always found to 

 be present on the spot where the chemical change or transforma- 

 tion is going on. Besides these general functions, the several 

 substances found in plants exercise also special functions in 

 reference to vegetable life and growth. Thus, oiitrogen is most 

 abundant in the sap of young plants, takes part in most of the 

 changes of organic compounds which go on in the sap, and fixes 

 itself, as the plant approaches maturity, in greatest abundance in 

 the seeds and in the green leaves. Nitrogenous manure alone 

 produces negative results. Potash and soda circulate in the sap, 

 influence chemical changes very much, and reside or fix them- 

 selves most abundantly in green and fleshy leaves, and in bulb- 

 ous roots. Sul2Jhuric acid is very influential in all chemical 

 changes, is found in most cases in those parts of the plant in 

 which soda and potash abound, and deposits a portion of its 

 sulphur wherever the compounds of nitrogen form a notable part 

 of the substance of the plant. Pliospwric acid exercises also 

 much influence over the chemical changes of the sap, and finally 

 fixes itself in greatest abundance in the seeds and other repro- 

 ductive parts of the plant. Soluble phosphates, from whatever 

 source derived, produce no difference, whether as dissolved guano, 

 coprolites, or bone ash. Lime is very important to healthy vege- 

 table growth, as practical experience has long testified. Among 

 other duties, it appears to accompany the phosphoric acid in the 

 sap of plants, and to deposit itself in combination with organic 

 acids in the leaves and bark, and with phosphoric acid in some 

 seeds and roots. Magnesia appears also to attach itself very 

 much to phosphoric acid in the sap, and fixes itself in combina- 

 tion with the acid principally in the seed. Chlorine — the chemi- 

 cal function of this substance in the sap is less understood even 

 than that of the other substances above-mentioned. It exists 

 chiefly in combination v/ith soda, and is much more abundantly 

 present in some plants, and in some parts of plants, than in 

 others. Though, as I have said, its immediate chemical function 

 in the plant is not understood, it forms a most important con- 

 stituent of the plant, in so far as the after uses of vegetables 

 in the feeding of animals are concerned. Silica exists in the sap 

 in a soluble form, and deposits itself chiefly in the exterior 



