814 FOOD OF PLANTS.—ORGANIC CONSTITUENTS, 
1. The Organic or Volatile Constituents, and their Sowrces.— 
The organic constituents of plants are, Carbon, Oxygen, Hydro- 
gen, Nitrogen, and Sulphur. The first three alone form the 
cellulose of which the cell-walls are composed (see pages 4 and 
23); while the protoplasmic contents of the cell are formed of 
compounds of these three elements with the two other organic 
constituents, namely, nitrogen and sulphur (see page 26). Phos- 
phorus is also regarded as a necessary constituent of these nitro- 
genous cell-contents (page 26); but it belongs to the inorganic 
constituents. 
These organic constituents are required alike by every species 
of plant, hence the great bulk of all plants is composed of the 
same elements, although the proportion of these varies to some 
extent in the different species, and even in different parts of 
the same plant. The following table, by Johnston, indicates 
approximately the relative proportion of the organic and inorganic 
constituents of some of our vegetable food substances in 1,000 
parts, and of the different elements of which the former are 
composed. These substances were first dried at a temperature 
of 230° Fahr. :— 
Wheat. Oats. Peas. Hay. Turnips. Potatoes. 
Carbon. -2" 3s 455 507 465 458 429 441 
Bedroven..! <i: y) 88. 64 61 5U 56 58 
Oxyoen 2 sy «450 367 401 587 422 439 
Nitrogen . ae On) 22 42 15 li 12 
AGU anh tee 28 40 51 90 76 50 
We must now make a few remarks on each of the organic 
constituents, the sources from which they are derived, and the 
state in which they are taken up by plants. 
Carbon is the most abundant organic constituent, forming as it 
does from 40 to 60 per cent. of the weight of the entire dried sub- 
stance of different species of plants. That plants thus contain a 
large proportion of carbon may be conveniently proved by taking 
a piece of wood, the weight of which has been ascertained, and 
converting it into charcoal, which is impure carbon containing in 
its substance also a small quantity of the inorganic constituents 
or ash. The charcoal thus produced is of the same form as the 
piece of wood from which it was obtained, and when weighed 
it will be found to have constituted a large proportion of its 
original substance. As carbon is a solid substance and insoluble 
in water, it cannot be taken up in its simple state, for plants, as 
already noticed, can only take up their food as gas or vapour, 
or dissolved in water. In the state of combination, however, 
with oxygen, it forms carbon dioxide, which is always present 
in the atmosphere and the soil. Carbon dioxide is also soluble 
to some extent in water. Hence we have no difticulty in 
ascertaining the source of carbon and the condition and mode 
in which it is absorbed by the plant ; thus it is taken up essen- 
tially combined with oxygen in the form of carbon dioxide, 
