REVIEW OF THE PRINCIPLES OF NUTRITION. 109 



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

 thus allowing time to form by decomposition a Afresh 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 different mineral substances 

 in their composition. 



322. But when all these materials have been supplied to the 

 plant, still two other agents are requisite, without which the great 

 work of vegetation will not go on. These life-giving principles 

 are light and heat, both of which emanate in floods from the sun. 

 Under their influence the raw material is received into the ves- 

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

 cess which can be fully comprehended only by HIM whose 

 power is adequate to carry it on. 



323.- 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 this 

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

 is there subjected to the action of the chlorophylle (215, a), which 

 chiefly constitutes the apparatus of digestion. Here it is con- 

 centrated by exhalation and evaporation, sending off quantities 

 of pure water. Meanwhile the leaves are imbibing carbonic 

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

 oxygen to the air. 



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

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

 nitrogen, and minute portions of the mineral substances men- 

 tioned above. From this juice are elaborated the building 

 material of the vegetable fabric, and all its various products and 

 secretions. 



325. First, by the aid of light, chlorophylle is developed, cloth- 

 ing the plant in living green. Next lignin 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. Meanwhile gum, starch, and sugar, mitritive products 

 common to all plants, are also developed from the proper juice, 



