FOOD OF PLANTS AKD ITS SOURCES. 777 



that the sources are the earth and the air, more particularly the 

 latter ; and that they are principally absorbed in the fonns of 

 carbonic acid and water, the latter of which is not only food in 

 itself, as it is composed of oxygen and hydrogen, two of the 

 essential organic constituents of plants, but it is also the vehicle 

 by which other food is conveyed to them. 



2. The Inoraanic Constituents and their Sources. — The 

 amount of inorganic matter found in plants, as already observed 

 (p. 774), is very much less than that of the organic. The 

 inorganic matters are all derived from the earth in a state of 

 solution in water, and hence we see again, how important a 

 proper supply of water is to plants. While the organic consti- 

 tuents are the same for all plants, the inorganic constituents 

 vary very much in different plants. The inorganic consti- 

 tuents -differ from the organic also, in the following parti- 

 culars: — 1st, they are incombustible, and hence remain as ash, 

 when the organic constituents are dissipated by burning ; 2nd, 

 they are not hable to putrefaction, as is the case with them, 

 under the influence of warmth and moisture ; and 3rd, they can 

 be formed in the laboratory of the chemist, whereas the organic, as 

 a inile, have at present, only been produced in the tissues of the 

 living plant. 



The inorganic constituents of plants are the following : — 

 Chlorine, Bromine, Iodine, Fluorine, Sulphur, Phosphorus, 

 Sihcon, Potassium, Sodium, Calcium, Magnesium, Aluminium, 

 Manganesiura, Iron, Zinc, and Copper. Some of these appear 

 to be almost universally distributed in varying proportions, but 

 others are only occasionally met with. These various inorganic 

 constituents are not taken up in their simple states, but princi- 

 pally as soluble oxides, chlorides, bromides, fluorides, sulphates, 

 phosphates, silicates, &c. 



Although the amount of inorganic matter in plants is very 

 much smaller than that of organic, still this portion, however 

 small, is necessary to the life and vigorous development of most 

 plants, and probably of all; although, in certain Moulds, no 

 inorganic constituents have been detected. 



The inorganic constituents of plants are of great importance in 

 an agricultural pomt of view, as it is to their presence or absence, 

 the quantities contained, and the solubility or insolubility of 

 their compounds, which are present in a particular soil, that it 

 owes its fertility or otherwise, and its adaptability of growing 

 with success one or another kind of plant. 



notation of Crops. — The principle of the rotation of crops in 

 agriculture is founded upon the fact of different plants requiring 

 different inorganic compounds for their growth ; and hence, a 

 particular soil which is rich in materials necessary for some 

 plants, may be wanting or deficient in those required by others. 

 (See also Excretions of Boots, p. 732.) Thus, Wheat or any 



