PAET I. 



OF THE CHEMICAL PROCESSES IN THE NUTRITION OF VEGETABLFS- 



CHAPTER I. 



01' THE CONSTITUENT ELEMENTS OF PLANTS. 



THE ultimate constituents of plants are 

 those which form organic matter in general, 

 namely, Carbon, Hydrogen, Nitrogen, and 

 Oxygen. These elements are always pre- 

 sent in plants, and produce by their union 

 the various proximate principles of which 

 they consist. It is, therefore, necessary to 

 be acquainted with their individual charac- 

 ters, for it is only by a correct appreciation 

 of these that we are enabled to explain the 

 functions which they perform in the veget- 

 able organization. 



Carbon is an elementary substance, en- 

 dowed with a considerable range of affinity. 

 With oxygen it unites in two proportions, 

 forming the gaseous compounds known 

 under the names of carbonic acid and car- 

 bonic oxide. The former of these is emit- 

 ted in immense quantities from many vol- 

 canoes and mineral springs, and is a product 

 of the combustion and decay of organic 

 matter. It is subject to be decomposed by 

 various agencies, and its elements then ar- 

 range themselves into new combinations. 

 Carbon is familiarly known as charcoal, but 

 in this state it is mixed with several earthy 

 bodies ; in a state of absolute purity it con- 

 stitutes the diamond. 



Hydrogen is a very important constituent 

 of vegetable matter. It possesses a special 

 affinity for oxygen, with which it unites and 

 forms water. The whole of the phenomena 

 of decay depend upon the exercise of this 

 affinity, and many of the processes engaged 

 in the nutrition of plants originate in the 

 attempt to gratify it. Hydrogen, when in 

 the state of a gas, is very combustible, and 

 the lightest body known; but it is never 

 found in nature in an isolated condition. 

 Water is the most common combination in 

 which it is presented; and it may be re- 

 moved by various processes from the oxygen, 

 with which it is united in this body. 



Nitrogen is quite opposed in its chemical 

 characters to the two bodies now described. 

 Its principal characteristic is an indifference 

 to all other substances, and an apparent re- 

 luctance to enter into combination with 

 them. When forced by peculiar circum- 

 stances to do so, it seems to remain in the 

 combination by a vis inertia:; and very 

 slight forces effect the disunion of these 

 feeble compounds. 



Yet nitrogen is an invariable constituent 



of plants, and during their life is subject to 

 the control of the vital powers. But when 

 the mysterious principle of life has ceased 

 to exercise its influence, this element re- 

 sumes its chemical character, and materially 

 assists in promoting the decay of vegetable 

 matter, by escaping from the compounds of 

 which it formed a constituent. 



Oxygen, the only remaining constituent 

 of organic matter, is a gaseous element, 

 which plays a most important part in the 

 economy of nature. It is the agent em- 

 ployed in effecting the union and disunion 

 of a vast number of compounds. It is supe- 

 rior to all other elements in the extensive 

 range of its affinities. The phenomena of 

 combustion and decay are examples of the 

 exercise of its power. 



Oxygen is the most generally diffused 

 element on the surface of the earth ; for, 

 besides constituting the principal part of the 

 atmosphere which surrounds it, it is a com- 

 ponent of almost all the earths and minerals 

 found on its surface. In an isolated state it 

 is a gaseous body, possessed of neither taste 

 nor smelL It is slightly soluble in water, 

 and hence is usually found dissolved in rain 

 and snow, as well as in the water of running 

 streams. 



Such are the principal characters of the 

 elements which constitute organic matter ; 

 but it remains for us to consider in what 

 form they are united in plants. 



The substances which constitute the prin- 

 cipal mass of every vegetable are com- 

 pounds of carbon with oxygen and hydro- 

 gen, in the proper relative proportions for 

 forming water. Woody fibre, starch, sugar, 

 and gum, for example, are such compounds 

 of carbon with the elements of water. In 

 another class of substances containing car- 

 bon as an element, oxygen and hydrogen are 

 again present; but the proportion of oxygen 

 is greater than would be required for produc- 

 ing water by union with the hydrogen. The 

 numerous organic acids met with in plants 

 belong, with few exceptions, to this class 



A third class of vegetable compounds 

 contains carbon and hydrogen, but no oxy- 

 gen, or less of that element than would be 

 required to convert all the hydrogen into 

 water. These may be regarded as com- 

 pounds of carbon with the elements of 

 water, and an excess of hydrogen. Such 

 are the volatile and fixed oils, wax, and the 

 resins. Many of them have acid characters. 



The juices of all vegetables contain or- 

 ganic acids, generally combined with the 



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