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XXXVI.— 0» the Food of Plants. By Dr. Fownes. 
PRIZE ESSAY. 
[It may be pi-nper to reniciik tliat althougli circumstances have prevented the pub- 
lication of this Essay until the present time, it was written in the month of February, 
1842, and the prize adjudged in the following December. No alterations of any 
importance have since been made. — Geo. Fownes.] 
The subjects proposed by the Society for discussion are the 
following ; — 
1. The sources from which plants derive the elements of 
which they are composed. 
2. The mode in which farmyard dung strengthens the growth 
of agricultural crops. 
3. The mode in which other manures, either singly or com- 
bined, act on vegetation. 
It is obvious that the answer to the first of these questions 
must afford the key to the solution of the others. 
The inquiry very naturally divides itself into the following 
sections : — • 
1 . The history of soils, their origin and chemical nature. 
2. The structure and composition of plants, more especially 
the latter ; and 
3. The nature of the materials furnished to them as food by 
the earth and the atmosphere, and the modifications of these 
supplies by the agency of man. 
The Origin and Composition of Soils. 
The hard and rugged granite rock ; the black and shapeless 
masses, the residue of the fiery torrents of melted matter which 
still occasionally pour from the volcanic vents yet existing in 
many parts of the world ; the still more refractory limestone ; all, 
after a certain and very variable lapse of time, undergo a change 
which, commencing at the surface and travelling inwards, gra- 
dually covers them with a pulverulent or sandy matter, retentive 
of moisture, and capable of receiving the roots of plants and 
ministering to their existence. This process is called "disin- 
tegration," and is one partly chemical, partly mechanical. The 
solvent })owers of water, more especially when charged with car- 
bonic acid, which always happens to a greater or less extent ; the 
excretions from the roots of the first coming plants themselves ; 
the expansion of water by freezing ; the insinuation of the rootlets 
of plants into the crevices so formed, and their subsequent enlarge- 
ment, are probably the chief agencies directed to this end, and 
the result of the whole is the production of a soil, whose depth 
will be principally dependent on two circumstances, viz. the 
nature of the rock, and the length of time during which this 
