26 The Garden. 



while the tissue of the rootlets is especially adapted to absorb 

 liquids, and is incapable of talcing in solid matter^ however 

 minutely divided. Let these facts be borne in mind while pre- 

 paring your soils and manures. 



The oxygen and hydrogen required by plants is probably de- 

 rived principally from water. 



The nitrogen is obtained mostly by the decomposition of 

 ammonia (hartshorn), a compound of hydrogen and nitrogen, 

 always produced when any animal and almost any vegetable 

 substance decays. It is dissolved in water, absorbed by porous 

 substances in the soil, and thus furnished to the roots of plants. 



The source of the carbon, which forms much the larger por- 

 tion of the bulk of plants, is still to be sought. Carbon itself 

 is a solid, absolutely insoluble in water, and therefore not avail- 

 able. The chief, if not the only fluid composed of carbon, 

 naturally presented to the plant, is that of carbonic acid gas, 

 which consists of carbon united with oxygen. This gas makes 

 up, on the average, one two-thousandth of the atmosphere, 

 from which it may be directly absorbed by the leaves; but, 

 being freely soluble in water, up to a certain point, it must be 

 carried down by the rain and imbibed by the roots. The car- 

 bonic acid of the atmosphere is, therefore, the great source of 

 carbon for vegetation. Carbonic acid is also produced in small 

 quantities by the action of manures in the soil. 



The carbonic acid absorbed is decomposed in the leaves by 

 the action of solar light; the carbon being retained and the 

 oxygen thrown off — beautifully reversing the process of animal 

 respiration, and thus preserving the proper balance in the 

 atmosphere. 



The mineral matters which form the inorganic constituents 

 of plants are all either soluble in water, or in the acids or alka- 

 lies mixed with it, and are therefore readily absorbed by the 

 roots. 



The following analysis of wheat will give the reader an idea 

 of the principal mineral constituents of plants generally, as to 

 the number of their elements; their proportion will vary 



