FOOD AFTER GERMINATION, 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 aqueculture, 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 
aqueculture, 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. 
