iiCviEW OF THE PRINCIPLES OF KUTRITION. 10& 



321. Soils are often improved by lyiug fallow for a season, 

 thus allowing time to form by decomposition a fresh supply of 

 •that particular ingredient which had been exhausted by previous 

 •crops. On the same principle is explained the beneficial effects 

 of a rotation of such crops as require dififerent mineral substances 

 in tl eir composition. 



31. 2. But when all these materials have been supplied to the 

 plan :, still two other agents are requisite, w-ithout which the great 

 wor^; of vegetation will not go on. These life-giving principles 

 ux^iight and heat, both of w^ich emanate in floods from the sun. 

 Under their influence the raw iiiaterial is received into tlie ves- 

 sels of the plant, and assimilated to its own substance, — a pro- 

 -cess Which can be fully comprehended only by Him whose 

 pow3r is adequate to cany it on.; 



3S3. Under the influence of solar light, and a temperature 

 above the freezing point,, water is imbibed by the roots and 

 -raised into the tissues of the stem, dissolving, as it passes, small 

 portions of gum or sugar previously deposited there. In tliis 

 state it is crude sap. But passing on it enters the leaves, and 

 is there subjected to the action of the chlor&pkylh (215, a), which 

 ■chiefly constitutes the apparatus of digestion. Here it m eon- 

 •ceni .-ated by exhalation and evaporation, sending off" quantities 

 of pare water. Meanwhile the leaves are imbibing carbonic 

 acid,, decomposing it, retaining the carbon, and returning pure 

 (Gxygen to the air. 



324. Thus elaborated, the sap is now termed the proper. 

 JUIC3, and consists of course of carbon and water, with a little 

 nitrcgen, and minute portions of the mineral substances men- 

 tioned above. ) From this juice are elaborated the bwlding 

 maUrial of the vegetable fabric, and sll its various products and 

 sec." etions. 



3 :.5. First,,by the aid of light, clilorophylle is developed, cloth- 

 ing the plant in living green. Next Ugnin is produced, the 

 •peculiar principle of tissue, whether cellular, vascular, or woody, 

 consisting of carbon with the exact elements of water, viz. oxy- 

 gen and hydrogen. 



326. Meanwliile gum, starch, and sugar, nutritive products 

 common to all plants, ai-c also developed from the proper juice, — . 



