REVIEW OF THE PRINCIPLES OF NUTRITION. 161 



which lies just below the ordinary soil is moved and subjected to atmospheric in- 

 fluence. The subsoil, with less organic matter, contains often soluble fertilizing 

 earths which may thus be rendered available for the use of plants. 



845. The object of manuring is mainly to increase the quantity of organic 

 matter, or to restore to the soil those qualities which have been taken away by the 

 crops. By various amendments (as gypsum, lime, charcoal) ammonia is strongly 

 attracted from the air and yielded again to vegetation. Marl promotes the decom- 

 position of the soil, and ashes add to the potassa — a substance which also exists 

 naturally in soils, being derived from the decomposition of the rocks which contain 

 it, as granite, clay-slate, basalt, etc. 



846. Bone manure is rich in the phosphates indispensable in the formation of 

 albumine, gluten, and other blood-making qualities of fruits. The mineral phos- 

 phate of lime, bone-chalk, etc., are of the same nature. 



847. Guano is a manure whose great value depends upon its abundant nitrates 

 and ammoniacal salts. It is the excrement of sea-fowl which has for ages accumu- 

 lated in vast deposits on certain coasts and islands of South America and Africa 



848. Fallow ground. 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. 



849. Light and heat. After 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 arc light 

 and heat, both of winch emanate in floods from the sun. Under their 

 influence the raw material is received into the vessels of the plant and 

 assimilated to its own substance — a process which can be fully compre- 

 hended only by Him whose power is adequate to carry it on. 



850. Digestion. Under the influence of solar light and a temperature above the 

 freezing point, water holding various earths in solution is imbibed by the root3 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. Passing on it enters 

 the leaves, and is there subjected to the action of the chlorophylle (§ 657) which 

 chiefly constitutes the apparatus of digestion. Here it is concentrated by transpi- 

 ration, sending off quantities of pure water. Meanwhile the leaves are imbibing 

 carbonic acid from the air, decomposing it, retaining tho carbon, and returning pure 

 oxygen. Thus elaborated, the sap is now called 



851. The proper juice, consisting evidently of carbon and water, 

 with a little nitrogen and minute portions of the mineral substances 

 mentioned above. From this, the vital fluid, are elaborated the build- 

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

 secretions. 



852. Protein, or protoplasm, the substance of the primordial utricle, analogous 

 m composition to the living tissues of animals, and containing nitrogen, is organized, 

 first of all, from this vital fluid. Cellulose (or lignin) next, the peculiar principle of 

 vegetable tissue, whether cellulal, vascular, or woody, consisting of carbon with tho 



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