328 



RESPIRATION. 



their respiration deteriorate the air to the ex- 

 tent of removing in a century the 8000th part 

 of the oxygen in the atmosphere, is to make a 

 supposition very much beyond the truth.* 



Respiration of plants, The results of the 

 chemical actions between the atmospheric air 

 and the vegetable kingdom, are chiefly influ- 

 enced by the presence or absence of light, 

 and the condition of the plants at the time. 

 When a plant is surrounded by the ordinary 

 atmospheric air, and exposed to the sunshine, 

 the green parts of the plant, and especially the 

 leaves, decompose the carbonic acid contained 

 in the atmosphere, seize upon the carbon, and 

 liberate the oxygen ; while the same plant in 

 the dark, not only ceases to decompose car- 

 bonic acid, but actually exhales into the sur- 

 rounding atmosphere a portion of this gas. 

 A quantity of nitrogen gas is also given off 

 by plants along with the oxygen. f Plants, 

 therefore, during exposure to light, purify the 

 air by removing carbonic acid and adding 

 oxygen, while during the night they, like 

 animals, deteriorate the air by exhaling car- 

 bonic acid gas. As, however, the quantity of 

 oxygen gas liberated during the day from the 

 decomposed carbonic acid is more than suffi- 

 cient to counterbalance the quantity of car- 

 bonic acid formed during the night, plants on 

 the whole must counteract, either entirely or 

 in part, the accumulation in the atmosphere 

 of the carbonic acid gas formed by the respi- 

 ration of animals, and in various chemical 

 processes going on at the earth's surface. 

 Indeed, nearly the whole of the carbon which 

 enters so largely into the formation of the ve- 

 getable tissues, appears to be obtained through 

 the decomposition of the carbonic acid of the 

 atmosphere. 



The parts of a plant which are not of a 

 green colour, such as the roots, &c., absorb 

 oxygen from the atmosphere, and give out 

 carbonic acid gas even in the sunshine ; and 

 this process seems essential to the vigorous 

 growth of the plant. The flowers of a plant 

 also absorb oxygen, and exhale carbonic acid, 

 and the quantity of the latter gas evolved 

 during inflorescence is considerable. The 

 seeds of plants during germination also absorb 

 oxygen and give out carbonic acid.J The 



* Dumas' Essai de Statiqiie Chimique des Etres 

 Organises, p. 18, 3rd edit. 1844 ; and Dumas and 

 Boussingault in Aunales de Chim. et de Phys., torn, 

 iii. p. 288. 1841. 



f Daubeny, in Philos. Transactions of London for 

 1836, p. 149 ; and Professor Draper, in London, Edin- 

 burgh, and Dublin Philosophical Magazine, vol. 

 xxiii. p. 161. 1843. According to Draper, " when 

 the leaves of plants under the influence of light de- 

 compose carbonic acid gas, they assimilate all the 

 carbon, and a proportion of oxygen disappears, at 

 the same time they emit a volume of nitrogen equal 

 to that of the oxygen consumed." The greater part of 

 the nitrogen evolved comes, he believes, from the 

 decomposition of some nitrogenized constituent of 

 the leaf. 



J The animalcula, especially those of a green 

 colour, seem to exert the same effects upon the at- 

 mospheric air under the influence of light as the 

 green parts of plants. Vide observations of Morren 

 and 'Who'ler, already referred to ; and also Ehren- 

 berg, in Poggendorff's Aiuialen, band Ivii. S. 311. 



Fungi evolve carbonic acid gas in large quan- 

 tity from all parts of their structure, and at 

 all periods of their growth, even when exposed 

 to a bright sunshine, and these plants derive 

 their supply of carbon from the soil in which 

 they grow.* It is also maintained that a 

 quantity of oxygen is absorbed by the surface 

 of plants during spring and summer, to assist 

 in the elaboration of their acids, resins, and 

 volatile oils. We thus perceive that the che- 

 mical actions between the atmospheric air 

 and plants are varied, and differ in some 

 important respects from those that occur in 

 animals. Attempts have been made to show 

 that the respiratory function is essentially the 

 same in these two great divisions of the or- 

 ganic kingdom ; that the fixation of carbon 

 and the liberation of the oxygen gas by the 

 leaves, and other green parts of plants during 

 their exposure to the rays of the sun, form a 

 part of their digestive process ; while the evo- 

 lution of carbonic acid, which proceeds during 

 the day as well as during the night, from seeds 

 during germination, from the flowers, from 

 the surfaces not coloured green, and also, it 

 is asserted, partly from the leaves, is their 

 true respiratory process.^ According to 

 others, if the actions of the juices upon the 

 atmospheric air, by which they are changed 

 from the crude to the fully elaborated sap, 

 and rendered fit for the nutrition of the plant, 

 constitute the function of respiration, then 

 the green surfaces, and especially the leaves, 



* Marcet (Bibliotheque Universelle de Geneve, 

 (Sciences et Arts,) torn. Ivii. p. 393. 1834 ; and An- 

 nales de Chim. et de Phys., torn. Iviii. p. 407. 1835) 

 ascertained that Fungi, when confined in a limited 

 quantity of air for some time, disengage a larger 

 quantity of carbonic acid gas than could have been 

 formed by the combination of carbon with the oxy- 

 gen which has disappeared from the air : that when 

 confined in nitrogen gas, a small quantity of car- 

 bonic acid is evolved, and in some cases a small 

 quantity of nitrogen is absorbed ; and that when 

 confined in oxygen gas a larger quantity of this gas 

 is absorbed than what is sufficient to constitute the 

 carbonic acid gas evolved, and that this is replaced, 

 at least in part, by a quantity of azote disengaged 

 from the plants. We thus perceive that if certain 

 of the lower organized bodies, generally regarded as 

 belonging to the animal kingdom, effect the same 

 changes upon the atmospheric air by their respira- 

 tion as the higher vegetables do, there are, on the 

 other hand, certain of the lower organized plants 

 that resemble in this respect the higher organized of 

 the animal kingdom. Other crj^ptogamic plants 

 having a green colour, such as the Ferns and Alga?, 

 liberate oxygen gas when exposed to the sunshine. 

 Vide Morren's Experiments on Alga?, already re- 

 ferred to ; and those of Daubeny, upon Ferns and 

 Algse, in London Philos. Transact, vol. xlii. p. 166. 

 1836. 



f Burnett, in the Journal of the Royal Institution 

 of Great Britain, vol. i. p. 83. 1831. Mr. Burnett 

 also maintains that the analogy further holds good 

 " between the functions of respiration and digestion 

 in animals and plants, for to both is carbonic acid 

 deleterious when breathed, and to both is it invigo- 

 rating to the digestive system when absorbed as 

 food," p. 100. Professor Draper (London and Edin. 

 Philos. Magazine, 1844) proceeds still farther, and 

 asserts that the whole of the action of the leaves 

 upon the atmospheric air constitutes a true digestive 

 and not a respiratory function. 



