FOOD OF PLANTS IN GENERAL. 481 



question of their nourishment is involved, may be divided into 

 two classes, one containing no nitrogen, and the other containing 

 this element. The first class may be divided into three groups : 

 one in which, together with carbon, hydrogen and oxygen are 

 found in the proportion requisite to the formation of water (dex- 

 trine, &c.) ; a second, in which oxygen is present in superfluity 

 (vegetable acids) ; and a third, in which it is found in very small 

 proportions, or in which it is altogether absent (the oils). The 

 second class (the protein compounds) contains, together with the 

 four organic elements, sulphur and phosphorus. Hydrogen and 

 oxygen are always present in sufficient quantity in plants in the 

 form of water, without which no vegetation is possible. Carbon 

 is furnished from the carbonic acid derived from the processes of 

 burning and respiration, from putrefaction and from volcanic 

 eruptions ; all of which render it to the atmosphere from which it 

 is received by the vegetable world. Nitrogen is taken up in 

 the form of ammonia, or the salts of ammonia, which are found 

 during the commencement of the processes of combustion, re- 

 spiration, putrefaction, and during the eruptions of volcanoes. 

 Suphur and phosphorus are yielded probably from phosphuretted 

 and sulphuretted hydrogens. The last is formed whenever pro- 

 tein compounds containing sulphur putrify, and whenever organic 

 matter is decomposed in contact with the sulphates, and also during 

 volcanic action. 



In the foregoing paragraphs, I have shown how plants over the whole 

 earth, in order to obtain their food, need the mediation of the inorganic 

 world ; for through it alone organic substances minister to their exist- 

 ence, for plants cannot receive their nourishment in the form of or- 

 ganised, but in the form of unorganised matter. In this place we must 

 afford especial proof that the individual elements are taken up in the 

 inorganic, and not the organic, form, and also point out the essential 

 sources of these combinations. I may, for the time, lay aside any discus- 

 sion respecting the absorption of hydrogen and oxygen, for no plant 

 can vegetate without water ; and usually much more water is received 

 into the substance of a plant than is requisite as the mere vehicle of 

 hydrogen and oxygen. But I must notice sulphur and phosphorus, on 

 account of their union with protein to produce albumen, fibrine, and 

 casein. All the observations which have already been made on organic 

 substances in general will apply to carbon and nitrogen : there is, how- 

 ever, much of a special nature, and in relation to carbon some interest- 

 ing facts to be brought forward ; whilst, with regard to nitrogen, and 

 especially on the sources of ammonia, much has yet to be added. 



1. Carbon. Over the whole face of the earth, with the exception 

 perhaps of some few savage tribes little known to us, man is acquainted 

 with the use of fire in the preparation of his food, and in the colder 

 zones for the purpose of communicating warmth ; whilst in the torrid 

 zones it is used also to keep off wild beasts and deleterious insects. 

 Civilised nations employ it also for a variety of purposes in the arts. 

 Amongst the civilised nations of the temperate zones, combustible 

 materials are used indeed with economy, and especially in cases where 

 numbers live together in one house, since the fire which will warm one 



I I 



