794 PHYSIOLOGY. 



ing particulars : — 1st, they are incombustible, and hence remain 

 as ash, when the organic constituents are dissipated by burning; 

 and, 2nd, they are not liable to putrefaction, as is the case with 

 them, under the influence of warmth and moisture. 



The inorganic constituents of plants are the following: — 

 Chlorine, Bromine, Iodine, Fluorine, Silicon, Potassium, Sodium, 

 Calcium, Strontium, Magnesium, Aluminium, Manganesium, 

 Iron, Zinc, Titanium, Lithium, Caesium, Kubidiura, Arsenic, and 

 Copper. Some of these appear to be almost universally dis- 

 tributed in varying proportions, but others are only occasionally 

 met with. These various inorganic constituents are not taken 

 up in their simple states, but 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 point of view, as it is to their presence or absence, 

 their relative quantities, and the solubility or insolubility of 

 their compounds, in a particular soil, that it owes its fertility or 

 otherwise, and its adaptability of growing with success one or 

 another kind of plant. 



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

 agriculture is founded iipon the fact of different plants reqiiiring 

 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 Excretion by the Root, p. 745.) Thus, Wheat or any 

 cereal crop requires more especially for its proper growth a full 

 supply of silica and phosphates ; hence it will onl}^ floui'ish in 

 a soil containing the necessary amount of such substances. As 

 growth proceeds, these constituents are absorbed in a state of 

 solution by the roots, and are applied to the requirements of the 

 plants. When the grain is ripe, it is removed as well as the 

 straw, and the silica and phosphates obtained from the soil will 

 be also removed with them : the result of this is necessarily, 

 except in fertile virgin soil, that these ingredients will not be 

 then contained in the soil in sufficient quantities to support 

 immediately the grow^th of the same class of plants; but by 

 growing in a soil thus exhausted by Wheat another crop of a 

 different kind, such as Clover, Peas, Beans, &c. &c., which 

 requires either altogether different substances, or a different 

 amount, or distinct combinations of the same substances, we may 

 obtain a profitable crop, while at the same time certain chemical 

 changes will go on in the soil, and other ingredients will be 

 taken up from the atmosphere, &c., by which the land will be 

 again adapted for the growth of Wheat. 



