rOOD OF PLANTS AOT) ITS SOURCES. 775 



1. The Organic Constituents and their Sources. — The organic 

 constituents of plants are, Carbon, Oxygen, Hydrogen, and 

 Nitrogen. The first three alone form the cellulose of which 

 the cell-walls are composed (see p. 9), and are therefore to be 

 considered as constituting by themselves the proper fabric of 

 the plant ; while the protoplasmic contents of the cell are formed 

 of compounds of these three elements, with the fourth organic 

 constituent — nitrogen. It would appear also, that two other ele- 

 ments, namely, Sulphur and Phosphorus, are also necessary 

 constituents of these nitrogenous cell-contents. 



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 difierent 

 parts of the same plant. The following table, by Johnston, 

 indicates the relative proportion of the organic and inorganic 

 constituents of some of our vegetable food substances in 1000 

 parts, and of the different elements of which the former are 

 composed. These substances were first dried at a' temperature 

 of230°rahr:— 



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 element which forms the largest proportion of 

 all plants ; its amount varies in different species from 40 to 60 

 per cent. That plants thus contain a large proportion of car- 

 bon 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 ashes. 

 The charcoal thus produced is of the same shape 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 sub- 

 stance. As carbon is a soHd substance and insoluble in water, 

 it cannot be taken up in its simple state, as plants, as already 

 noticed (see pp. 731 and 774) can only take up their food as gas 

 or vapour, or dissolved in water. In the state of combination 

 however, with oxygen, it forms carbonic acid, which is always 

 present in the atmosphere and the soil. Carbonic acid is also 

 soluble to some extent in water. Hence we have no difficulty 

 in ascertaining the source of carbon and the condition and modes 

 in which it is absorbed by the plant ; thus it is taken up com- 

 bined with oxygen in the form of carbonic acid, from the air 



