REVIEW OF THE PRINCIPLES OF NUTRITION. 109 
$21. Soils are often improved by lying 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 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 goon. 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 prorEer 
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 butlding 
material of the vegetable fabric, and all its various products and 
secretions. 
325. First, by the aid of light, chlorophyile 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, nutritive products 
common to all plants, are also developed from the proper juice,— 
