774 PHYSIOLOGY. 



the requirements of the plant, but is but little altered from the 

 condition in which it was absorbed ; and lastly, our attention 

 will be briefly directed to the functions of development and 

 secretion. 



Section 1. Food of Plants, and its Sources. 



The various substances required as food can only be ascertained 

 when we know the elementary composition of the parts and 

 products of plants; for as plants have no power of forming these 

 elements for themselves, they must have derived them from 

 external sources, and hence an acquaintance with them is at 

 once an indication to us of the nature of their food. 



As plants are commonly fixed to the soil, they must gain 

 their food from it, or from the air which surrounds them. 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 (fg. 237), derive their food entirely from 

 the air by which they are surrounded (see p. 124) ; while 

 Parasites (Jigs. 238 and 239) are essentially different from both 

 Epiphytic and ordinary plants, since their food, instead of being 

 derived entirely from inorganic materials, which are afterwards 

 assimilated in their tissues, as is the case with them, is obtained 

 entirely or partially from the plants upon which they grow, — 

 that is, in an already assimilated condition. (See p. 125.) 



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 at once enables 

 us to distinguisli the comparative proportion of these, and 

 acquaints us with one of their distinctive peculiarities. Thus, 

 if we take a piece of wood, or a leaf, or any other part of a 

 plant, and bum 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 

 tlie organic, and tlie latter the inorganic or earthy constituents. 

 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 organic products and secretions. 

 The relative ])roportion of the organic and inorganic consti- 

 tuents varies in different plants, but as a general rule, the former 

 constitute from 89 to 99 parts, while the latter form from one 

 to about eleven parts in every hundred, of all plants. 



