POOD ABTEE GKEMINATIOIT. 329 



Acid (nitrates) are commonly the chief, and may be the 

 only supply of this element. 



The other proximate principles, viz. pectose, the fats, 

 the alkaloids, and the acids, are built up from the same 

 food-elements. In all cases the steps in the construc- 

 tion of organic matters are unknown to us, or subjects of 

 uncertain conjecture. 



The carbohydrates, albuminoids, etc., that are organized 

 in the foliage, are not only transformed into the solid tis- 

 sues of the leaf, but descend and diffuse to every active 

 organ of the plant. 



The plant has within certain limits a power of selecting 

 its food. The sea-weed, as has been remarked, contains 

 more potash than soda, although the latter is 30 times 

 more abundant than the former in the water of the ocean. 

 Vegetation cannot, however, entirely shut out either ex- 

 cess of nutritive matters or bodies that are of no use or 

 even poisonous to it. 



The functions of the Atmosphere are essentially the 

 same towards plants, whether growing under the condi- 

 tions of aquseculture, or under those of agriculture. 



The Soil, on the other hand, has offices which are peculiar 

 to itself. We have seen that the roots of a plant have the 

 power to decompose salts, e. g. nitrate of potash and 

 chloride of ammonium (p. 170,) in order to appropriate 

 one of their ingredients, the other being rejected. In 

 aquseculture, the experimenter must have a care to re- 

 move the substance which would thus accumulate to the 

 detriment of the plant. In agriculture, the soil, by virtue 

 of its chemical and physical qualities, renders such reject- 

 ed matters comparatively insoluble, and therefore innoc- 

 uous. 



The Atmosphere is nearly invariable in its composition 



at all times and over all parts of the earth's surface. Its 



power of directly feeding crops has, therefore,, a natural 



limit, which cannot be increased by art. 

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