No. 12. Free accession of Air necessary to Decomposition. 



381 



hydrogen — have been separated, the carbon 

 retains the form of the tissues in great beauty 

 and perfection, so that a piece of charcoal will 

 indicate the character of the wood from which 

 it was made, nearly as well as would a sec- 

 tion of ail unburnt branch. On the other 

 hand, in proportion as the tissues of the plant 

 are defined in carbon, do we find them defi- 

 cient in firmness of structure. When, 

 therefore, we consider the large quantities 

 of carbonic acid extricated by the respiration 

 of animals, and by the immense amount of 

 combustion of coal which is constantly going 

 on in our large towns, there would seem no 

 difficulty in understanding how it may be 

 supplied to plants; but so vast is the extent 

 of the atmosphere through which the carbonic 

 acid has to be diffused, that any given bulk of 

 air only contains about 1-lOOOth part of this 

 gas. Hence, it might be supposed impossible 

 for the gigantic mass of carbon contained in 

 the wood of a wide-spreading forest to have 

 been derived chiefly, if not entirely from this 

 source, and such is seen to be the case ; for 

 although the soil may contain carbon, none 

 of it is taken up in a solid form ; and its 

 quantity rather increases than diminishes in 

 the course of years. 



Oxygen is contained largely in plants; 

 and the presence of it in the air which sur- 

 rounds them, is very necessary to their 

 healthful existence, chiefly as affording the 

 means by which, as already explained, the 

 superfluous carbon is removed. This ele- 

 ment is equally necessary to animals, and it 

 constitutes about a fifth part of the air we 

 breathe. A portion of this air is dissolved, 

 as it were, in water, and it is in this manner 

 that fish and other aquatic animals as well 

 as plants are supplied with oxygen. Most, 

 if not all, however, of the oxygen which is 

 contained in vegetable substances, is taken 

 up by them, either in combination with car- 

 bon, or in unison with hydrogen — a body 

 with which it forms water. 



Hydrogen is also contained largely in 

 plants; and in most of the substances into 

 whose composition it enters, it is combined 

 with oxygen nearly in the same proportion as 

 in water; although it is probable that a small 

 quantity is introduced with nitrogen in the 

 form oi ammonia — the pungent gas which 

 gives strength to hartshorn, smelling salts, 

 &c. — we may regard the water introduced 

 into the substance of plants by their roots, 

 and also in part absorbed by their general 

 surface, as the chief source of this element, 

 as well as if the oxygen continued in the 

 vegetable structure. 



Nitrogen has not been commonly regarded 

 as an important element of the vegetable 

 structure, but it has been lately shown to ex- 

 ist largely in the growing parts of plants; 



and there seems reason to believe its pres- 

 ence to be essential to the increase of their 

 fabric by the formation of new parts. It is 

 an important ingredient in the substance 

 called gluten, which exists largely in the 

 seeds of the various kinds of corn or grain, 

 and most of all, in wheat ; and it is in part on 

 this account that wheat-bread is the most nu- 

 tritious of all vegetable substances ordinarily 

 used as food, since it approaches nearer in 

 composition than almost any other to animal 

 flesh, which contains a much larger proportion 

 of nitrogen than exists in most vegetable sub- 

 stances. It is, indeed, on account of their 

 entire deficiency in nitrogen, that gum, su- 

 gar, and other similar products are not fit to 

 maintain animal life by themselves. Nitro- 

 gen constitutes four fifths of the atmosphere, 

 but it does not seem to be taken in by the 

 plant in its simple form ; but this gas with 

 hydrogen, forms ammonia, of which a minute 

 quantity always exists in the atmosphere, be- 

 ing chiefly supplied to it by the decomposition 

 of animal matter ; and this is absorbed by the 

 soil and taken up by the roots; and it is in 

 the supply of ammonia which they yield, that 

 the principal benefit of animal manure seems 

 to consist. Treatise of Veg. Physiology. 



Free accession of Air necessary to 

 Decomposition. 



The decomposition of the vegetable matter 

 of the soil requires the free access of air to 

 every part of it; for if any substance, how- 

 ever rapid its tendency to decay, be com- 

 pletely secluded from the atmosphere, little 

 or no change in it can take place: it is on 

 this principle that various articles of food are 

 now preserved for subsequent use in tin cases 

 completely closed, and possess their perfect 

 flavour after exposure to every variety of 

 temperature for several years. Every parti- 

 cle of the soil needs to be surrounded with 

 oxygen for the production from it of carbonic 

 acid; and to procure this condition is one of 

 the chief objects, which is effected by tilling 

 and loosening the soil ; in this respect, there- 

 fore, it is manifest that a tenacious clayey 

 soil is inferior to all others, and its injurious 

 character can only be remedied by admixture 

 with other substances, or by laborious culti- 

 vation. This necessity of unimpeded access 

 of air to that part of the ground through 

 which the roots of plants are distributed is 

 shown in an interesting manner, when trees 

 are planted too deep in the soil, or when 

 their roots have been covered with an addi- 

 tional quantity of earth, when, if the tree be 

 old or sickly, it generally dies, but if young 

 and vigorous, it sends out a new set of roots 

 nearer" the surface, and the extension of the 

 old ones ceases. — Veg. Fhy. 



