INTROVUCTION. 19 
mother seed, the materials, as well as the mode of its nu- 
trition, are for the most part completely changed. Hence- 
forth the tissues of the plant and the cell-contents must 
be principally, and may be entirely, built up from purely 
inorganic or mineral matters. 
In studying the nutrition of the plant in those stages 
of its growth that are subsequent to the exhaustion of the 
cotyledons, it is needful to investigate separately the nu- 
tritive functions of the Atmosphere and of the Soil, for 
the important reason that the atmosphere is nearly con- 
stant in its composition, and is beyond the reach of human 
influence, while the soil is infinitely variable and may be 
exhausted to the verge of unproductiveness or raised to 
the extreme of fertility by the arts of the cultivator. 
In regard to the Atmosphere, we have to notice minutely 
the influence of each of its ingredients, including Water 
in the gaseous form, upon vegetable production. 
The evidence has been given in “ How Crops Grow,” which 
establishes what fixed earthy and saline matters are essential 
ingredients of plants. The Soil is plainly the exclusive 
source of all those elements of vegetation which cannot as- 
sume the gaseous condition, and which therefore cannot ex- 
ist in the atmosphere. The study of the soil involves a con- 
sideration of its origin and of its manner of formation. The 
productive soil commonly contains atmospheric elements, 
which are important to its fertility; the mode and extent 
of their incorporation with it are topics of extreme prac- 
tical importance. We have then to examine the signif- 
icance of its water, of its ammonia, and especially of its 
nitrates. These subjects have been recently submitted to 
extended investigations, and our treatise contains a large 
ainount of information pertaining to them, which has never 
before appeared in any publication in the English tongue, 
Those characters of the soil that indirectly affect the 
growth of plants are of the utmost moment to the farm- 
er. It is through the soil that a supply of solar heat, with: 
