FOOD OF PLANTS AND ITS SOURCES. 791 



they are surrounded, that is, from the soil, or from the air, or 

 from both. In by far the majority of cases, plants take up 

 their food, both from the air by their leaves in a gaseous or 

 vaporous state, and from the earth dissolved in water. No 

 plants have the power of taking up nutriment except in the 

 state of gas or vapour, or in a fluid state. Those plants which 

 are termed Epiphytes or Air Plants, as Orchids {fig. 230), 

 derive their food almost entirely from the air by which they are 

 surrounded (see p. 118) ; while Parasites {figs. 231 and 232) 

 essentially differ from both Epiphytic and ordinary plants, in 

 the fact that their food, instead of being derived entirely from 

 inorganic materials, which are afterwards assimilated in the 

 tissues, is obtained entirely or partially from the plants upon 

 which the}' grow, — that is, in an already assimilated condition 

 (seep. 119). _ 



The materials of which plants are composed, and which, as 

 stated above, are either derived from the air or the earth, or 

 more commonly from both, and which consequently constitute 

 their food, are of two kinds, called respectively the organic 

 and the inorganic. The process of burning enables us con- 

 veniently to distinguish, to a great extent at least, the compara- 

 tive proportion of these, and acquaints us with one of their dis- 

 tinctive peculiarities. Thus, if we take a piece of wood, or a leaf, 

 or any other part of a plant, and burn it as perfectly as we are 

 able, we find that the greater portion disappears in the form of 

 gas and vapour, but a small portion of the original substance 

 remains in the form of ash or incombustible material. The 

 former are termed the organic, and the latter the inorganic or 

 earthy constittoents. The term organic is applied because such 

 materials especially constitute the real fabric of the plant, and 

 are more essentially concerned in the formation of its proper 

 products and secretions. The relative proportion of the organic 

 and inorganic constituents varies in different plants, but, as a 

 general rule, the former constitute from 89 to 99 parts, while 

 the latter form from 1 to about 11 parts in every 100. 



1. The Organic or Volatile Constituetits and their Sources. — 

 The organic constituents of plants are. Carbon, Oxygen, Hydro- 

 gen, and Nitrogen. The first three alone form the cellulose of 

 which the cell-walls are composed (see p. 18), 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 elements, namely. Sulphur and Phosphon\s, are also 

 necessary constituents of these nitrogenous cell-contents. 



These organic constituents are required alike by every spe- 

 cies 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 different parts 



