FARMERS' REGISTER— VEGETABLE PHYSIOLOGY. 



171 



cellular passages, as we before mentioned, by means 

 of the successive contractions and dilatations of the 

 cells themselves. But we decidedly consider the 

 newly established principle of endosmose to affoi-d 

 a much better prospect of accounting for the fact. 

 Endosmose is that property of all membranaceous 

 substances, by wliicii two opposite currents are es- 

 tablished through their texture, whenever two 

 fluids of unequal densities are placed on opposite 

 sides of them. That the vital force must also be 

 employed in securing any lengthened continuance 

 of this action, is evident; for in every apparatus 

 constructed for the purposes of experiment, the ef- 

 fect must cease as soon as the two fluids have ac- 

 quired the, same density. Now, without attempt- 

 ing to account for the manner in Avhich the healthy 

 condition of the membrane is secured, Ave may ea- 

 sily imagine the constant developement of fresh 

 vesicles, and the continued secretion of fresh ma- 

 terials, to be sufficient to maintain the conditions 

 necessary for the establishment of a perpetual en- 

 dosmose during the lifetime of the plant, without 

 considering this property itself to be directly de- 

 pendant on the vital force. This property also ex- 

 plains the prodigious force with which the sap 

 rises in certain seasons of the year; a force suffi- 

 cient, as Hales determined, to support a pressure 

 equal to two atmospheres and a half, and five 

 times that by whicli the blood is pi'opelled in tlie 

 crural artery of the horse. The ascent of the sap 

 is the result of a compound action, partly depend- 

 ing on a force "a tergo" propelling it forward, and 

 partly on a force attracting it towards the foliace- 

 ous parts of the leaf, each, however, resulting from 

 the endosmose carried on by all parts of the cellu- 

 lar texture. 



Third period of nutrition. — When the sap has 

 arrived at the leaves, and at the other green parts 

 on the surface of the plant, a considerable portion 

 of its aqueous particles is transpired. A cabbage, 

 for example, transpires from a given surface se- 

 venteen times as much as a man by his insensible 

 perspiration. A small portion indeed, of this ef- 

 fect must be ascribed to the process of evaporation ; 

 but this is comparatively very trifling, and the 

 greater part must be attributed to the action of a 

 vital function. This is so decidedly remarkable 

 in the vegetable kingdom, that De Candolle pro- 

 poses for it the name of "exhalation," in order to 

 distinguish it from the less conspicuous effects ot 

 the insensible perspiration of animals. It is mani- 

 festly produced by the instrumentality of the sto- 

 mata, or glandular pores, seated on the gi-een parts 

 of plants, and which are more especially abundant 

 on the under surface of the leaf. Heat exerts a 

 trifling influence in producing an increase of ex- 

 halation, but light is the chief stimulant which de- 

 termines its extent. Plants do not exhale mois- 

 ture in the dark, and as they still continue to ab- 

 sorb a little, they soon become dropsical. The fluid 

 exhaled is nearly pure water, and consequently the 

 gap must become considerably altered by this cir- 

 cumstance alone, as the materials introduced in solu- 

 tion will now bear a higher proportion to the whole 

 quantity of water retained in the plant. This great 

 exhalation of the superabundant fluid may be consi- 

 dered analogous to the combined effects produced 

 both by the insensible perspii-ation and excremen- 

 titial rejections of animals. 



Fourth period of niUr-ition. — We have now ar- 

 uivedat the complicated phenomenon of vegetable 



"respiration," the most important of all the prO' 

 cesses which together constitute the function of nu- 

 trition. One circumstance in this process is strict- 

 ly in accordance with what takes place in the res* 

 piration of animals; the presence of oxygen being 

 equally essential to the life of the individuals of each 

 kingdom, though the ultimate results are diame- 

 trically opposite in eacii. In animals, the oxygen 

 inhaled unites with the superfluous carbon in the 

 blood, and the carbonic acid thus formed is ex- 

 haled into the atmosphere. In plants, a similar ef- 

 fect takes place by night; when the leaves and 

 other green parts inhale the atmosphere, whose 

 oxygen unites with the carbonaceous matters dis- 

 solved in the sap ; but the carbonic acid thus gene- 

 rated is, for the most part, retained in solution 

 within the plant, and not exhaled again. All the 

 colored parts of plants perform this function as 

 well by day as by night ; but the green parts al- 

 ways decompose carbonic acid by day, from whate- 

 ver source they may be able to derive it; and the 

 result of this decomposition is to fix the whole of 

 tlie carbon, and a small portion of the oxygen, in 

 the substance of the plant, and to exhale the rest 

 of the oxygen into the air. The chief supply of 

 carbonic acid provided for this purpose is that 

 which is introduced by the roots in a state of so- 

 lution ; but the small quantify universally present 

 in the atmosphere is also inhaled and decomposed 

 by the leaves. Tliat which is formed Avithin the 

 plant itself is cither retained in solution, or it is ex- 

 haled and disseminated in the atmosphere, Avhence 

 it may re-enter the plant by one or other of the 

 two methods just specified. Although the decom- 

 position of carbonic acid is always proceeding 

 during the day, yet it is never so rapid, nor so de- 

 cidedly appreciable, as Avhen the plant is exposed 

 to the direct rays of the sun. It is independent of 

 the presence of the stomata, Avhich Ave have alrea- 

 dy described as being the true exhaling organs of 

 the green parts ; and it is certainly effected by 

 means of the cellular texture of these same parts, 

 Avhose green tint must be ascribed to the result of 

 this very action. There can be no question that 

 the " decomposition" of the carbonic acid is the 

 direct operation of a vital function; but the " for- 

 mation" of this gas in the colored parts of plants, 

 at all times, and in the green parts by night, ap- 

 pears to be the result of a chemical action similar 

 to the ordinary process of decomposition in all 

 dead organized matter. If a plant be exposed to 

 the light in an atmosphere deprived of oxygen, it 

 soon dies, unless (Avhich is very remarkable) it be 

 enabled first to form a little oxygen by decomjws- 

 ing some portion of the carbonic acid Avithin it, 

 and thus to Impregnate the atmosphei-e Avith a suf- 

 ficient quantity to enable it to act as a further re- 

 source and constant stimulus for the formation ancj 

 deconqiosition of fresh supplies of carbonic acid. 



The result of all these compositions and decom- 

 positions of carbonic acid in the living plant, is the 

 fixation of the Avhole of that carbon Avhich is 

 (bund in the entire mass of vegetation on the sur- 

 face of tiie earth. Thus the atmosphere is conti- 

 nually being purified of every fresh addition of 

 this material Avith which comliustion, respiration, 

 and putrefaction are perpetually adulterating it. — 

 For though all living plants do themselves gene- 

 rate carbonic acid in the way Avhich Ave have spe- 

 cified, they also decompose much more than they 

 form, and this excess is considered to be sufficient 



