CHAP. XI. 
FOOD OF PLANTS. 
37 i 
table tissue; and, consequently, it is probable that the exhaus- 
tion of soil does not depend merely upon the destruction of 
carbonaceous matter, but also upon the consumption of the 
azotised matter contained in it. This is a most important 
fact to consider, in attempting to estimate the action of ma- 
nures. (Comptes rendus^ vi. 106.) M. Payen asserts that every 
nascent, or developing, organ contains nitrogen in abundance, 
and that, as a given organ developes, the azotised matter di- 
minishes in proportion to the unazotised, which by degrees 
becomes predominant. {Ibid. vi. 132.) It is, therefore, essen- 
tial for plants to be placed in such circumstances as may 
give them the power of assimilating nitrogen. 
This diminishes the complicated nature of the theory of 
manures, and the seeming impossibility of reducing it to any 
fixed and intelligible laws. But, ignorant as we are of most 
of the more obscure phenomena that are attendant upon 
vegetable life, unacquainted with the action of a large pro- 
portion of the principles that the chemist discovers among 
the tissue of plants, and incapacitated by our limited means 
of observation from watching any except the most obvious 
and general properties of living vegetable matter, we cannot 
expect, in such a state of things, to arrive at any precise ideas 
as to what kind of food or stimulants exercises the most ener- 
getic and wholesome influence upon plants. I accordingly 
feel no surprise at the statement of a friend of mine, well 
known alike for his agricultural skill, his chemical knowledge, 
and his remarkable good sense, ‘‘ that chemistry has hardly 
advanced the art of agriculture a single step, but that the 
latter remains, after all the investigations of the chemists, a 
mere empirical art.” 
Those who wish to understand the modern opinions con- 
cerning the action of manures (properly so called) should 
consult De Candolle’s Physiologie, p. 1278., and the papers 
of Payen, Boussingault, Thaer, &c. 
B B 2 
