FOOD OF PLANTS. 557 



according to circumstances, a nitrate or an ammonia salt, a salt of potash, 

 soda, lime, magnesia, and iron, while lime is necessary in the formation 

 of the cell-wall. These substances appear to act as ferments : for instance, 

 lime is stated to effect the conversion of cane-sugar into cellulose, and 

 to be influential in the transport of starchy materials in a soluble form. 



Sources of Nutriment. We have stated that (green) plants in 

 general acquire their nitrogenous food by their roots (from the 

 nitrates of the soil), and their carbonaceous food by their leaves. 

 The sources of the food are therefore the soil and atmosphere in 

 which plants grow ; and the inquiry presents itself at once as to the 

 form in which the food is supplied to and taken up by plants. 



How Plants get their Food. On the one hand, we know that 

 plants can absorb substances only in a liquid or gaseous form ; on 

 the other, we know that both the atmosphere and the soil contain 

 carbonic dioxide, water, and various nitrogenous compounds 

 soluble in the latter. The alkalies, earths, &c. exist only in the soil, 

 and in more or less abundance and in more or less soluble forms 

 in different cases. 



Observation teaches us that the simpler plants, such as the 

 Palmellea3, Lichens, many Mosses, &c., can grow upon bare rocks 

 or stones, and obtain their carbon, hydrogen, oxygen, and nitrogen 

 from the atmosphere alone ; and experiment shows that these are 

 supplied in the form of carbonic dioxide, water, and ammonia; the 

 substratum here supplies only the small proportion of mineral 

 substance that is required. Moreover it is possible to grow a 

 plant to maturity, and even to make it ripen its seed, in distilled 

 water containing in solution only the ash-elements of aquatic 

 plants, such as Confervas, &c. Similar growth may be obtained 

 by growing a plant in a watery solution of the necessary mineral 

 ; ingredients of the plants, together with a nitrate or an ammonia 

 salt, the excess of carbon in these cases being derived from the air. 

 Numerous and important results have been obtained by growing 

 plants in experimental solutions of this kind water culture ; and 

 . by their aid, as well as by field trials, it has become possible to 

 compound artificial manures adapted to the requirements of parti- 

 cular plants. 



Further, it is observed that, if a vegetation of this kind goes 

 on undisturbed for a lengthened period, the decay of successive 

 generations of plants leads to the accumulation of organic substance, 

 in vegetable mould, the material of which has been derived from 

 the atmosphere by the plants, but has not been consumed, i. e. 

 decomposed into its original forms of carbonic dioxide &c., by them 

 and their successors. 



From these facts it has been concluded, in the first place, and 



