SOURCES OF THE FOOD OF LIVING BKTNGH. 



structures , they greatly alter the form and properties of the 

 elements they take in ; but they can create nothing. It is easy 

 to say whence every particle, of which a living body consists, is 

 obtained by it ; for, by placing it in a variety of circumstances, 

 and observing the changes in its mode of life which these pro- 

 duce, we can determine the influence of each. Thus, an Animal 

 may be fed exclusively on some one kind of aliment, as for 

 instance sugar or gum ; and it is found that, however nutritious 

 when combined with others such an article may be, it has not 

 the power of supporting life for any length of time by itself, 

 unless it contain (which no single article of food except milk 

 does) all the substances required by the animal, for the right 

 maintenance of its structure. So, also, on the food of Plants we 

 may experiment, by placing them in different soils, and in dif- 

 ferent kinds of air, and by supplying them with variable quan- 

 tities of water ; until we have discovered, what is absolutely 

 necessary to their growth, what favours it, and what is super- 

 fluous or injurious. 



156. Before, however, we enter upon these inquiries, we 

 shall derive much guidance, from the knowledge of the substances 

 actually contained at any given time, in the Vegetable structure. 

 When we examine a seed, we find that it contains the germ of 

 the new being ; but that it principally consists (like the egg) of 

 a nutritious substance, prepared by the parent, for the support 

 and development of its offspring, until it is able to acquire food 

 for itself ; and it is by this means, as we shall hereafter see 

 (Chap, xii.), that the young plant is enabled to push its first 

 roots into the soil, and to elevate its first leaves into the air. By 

 the time it has done this, however, all that store of aliment is 

 exhausted ; and henceforth it is entirely dependent upon what it 

 acquires for itself. The lowly plant develops itself in progress 

 of years, by the wonderful power with which it is endowed, into 

 the gigantic tree ; increasing its weight, from a few grains to 

 many tons. Of what does its massive structure then consist ? 



157. The inorganic elements and mineral matter, composing 

 the solid earth on which we live, contain a certain number of 

 substances, which are termed simple ; because they cannot, by 



