BOTANY. 





33, and emit oxygrn gas and azote ; and that dur- 

 ing the niglit they absorb moisture, give out carbo- 

 nic acid gas, and absorb oxygen gas. 



From a view of the above statement, Dr Priestley 

 deduced the opinion of the purification of the atmo- 

 sphere by means of plants ; an opinion which at first 

 sceir.ed so consonant with the order and simplicity of 

 nature, that it was generally adopted, without a ut- 

 ficient examination of the facts, until Mr Elhs's In- 

 quiries into Vegetation and Respiration were laid be- 

 fore the public. This gentleman has the rare merit 

 of having calmly and deliberately ascertained the va- 

 lue of every fact, or supposed fact, by his own expe- 

 rience, and of having detected and exposed the errors 

 of his predecessors, with the candour, liberality, and 

 acuteness which characterise the genuine student of 

 nature. After a most laborious and careful set of 

 experiments on myrtles, beans, willows, and other 

 plants, Mr Ellis arrived at the general conclusion, 

 " that the oxygen gas of the atmosphere is convert- 

 ed into carbonic acid gas by the process of vegeta- 

 tion ; and that the bulk of the latter gas nearly or 

 exactly corresponds with that of the former ; and 

 consequently, they" (the experiments) " demonstrate 

 that the air is deteriorated by the growth of plants, 

 in the same manner as by the germination of seeds, 

 and that no part of the oxygenous portion of the at- 

 mosphere combines with the substance of the plant." 

 He also derives a very strong argument from the an- 

 atomical structure of the leaf: there is no proof of 

 the existence of vessels peculiarly fitted for absorb- 

 ing carbonic acid gas ; nor is it probable, that those 

 calculated for the reception and elaboration of the 

 circulating fluids can by any means receive an extra- 

 neous fluid ; nor can it be received by the exhaling 

 pores, as that would involve the impossibility of one 

 organ performing two distinct functions at the same 

 time. The common absorbents cannot be supposed 

 to receive it, as they, too, are otherwise employed. 

 If, then, this absorption is not carried on by any 

 living action, can it be the result of chemical affini- 

 ty ? This also is improbable ; for if it be the con- 

 sequence of chemical agency, all the laws of chemis- 

 try must be violated, as the structure of the leaf pre- 

 sents insurmountable obstacles to their action. The 

 natural inference from which is, that the carbonic 

 acid gas is formed exteriorly to the surface of the 

 leaf, and out of the very oxygen of the surrounding 

 atmosphere. Such are a few of the positive argu- 

 ments which Mr Ellis urges with much force in sup- 

 port of his opinion ; besides these, he analyses the 

 experiments and opinions of Priestley, Scheele, In- 

 genhousz, Woodhouse, Senebier, and Saussure ; and 

 he contends that the results obtained by them concur 

 in establishing the same position that he supports ; 

 and after having fully and satisfactorily shewn that 

 the carbonic acid gas i formed during the process of 

 vegetation in the shade, he proceeds to point out, as 

 a principal source of error, the neglect of the differ- 

 ence of the operations carried on in the sunshine and 

 in the shade ; and he states the distinction so con- 

 cisely and clearly, that we shall use his own words : 

 " From the facts which have now been stated, we 

 collect, that plant*, which vegetate in sunshine, re 

 quire always the presence of oxygen gas ; and that, 



by the act of vegetation, they constantly change this 

 oxygen into carbonic acid. We farther learn, that 

 carbonic acid enters plants, both with th<' fluids which " ~~ v 

 they absorb, and also, tinder certain circumstances, 

 in an elastic form j that this acid gas is conveyed to 

 the leaves, ajid is tlu-re decomposed by the joint ope- 

 ration of the plant and of solar light ; and that it is 

 from this source alone, that the oxygen gas afford. -d 

 by plants is derived. It likewise appears, that this 

 operation of affording oxygen is not properly a vege- 

 tative function, but .:nly a subordinate office, accom- 

 plished by the direct agency of the sun ; that it is 

 carried on in the cellular or parenchymatous struc- 

 ture, and not in the vascular system of the leaf ; and 

 that it may, and does exist with that function by 

 which oxygen is consumed, and which is essential to 

 the vegetation of the plant. Hence it is, that, when 

 plants are made to grow in closed vessels exposed to 

 the sun, the oxygen gas which is consumed by the 

 function of vegetation, is again restored by the de- 

 composition of the acid that is formed, and no change, 

 therefore, appears to be effected in the composition 

 of the air. But in situations, where the direct agency 

 of light is excluded, no decomposition of carbonic 

 acid is perceptible, and the air, therefore, soon be- 

 comes unfit to sustain vegetation. In its general na- 

 ture and effects, therefore, the function of vegetation 

 is precisely the same in sunshine and in the shade ; 

 for oxygen gas is alike necessary in both situations, 

 and ia in a similar manner converted into carbonic 

 acid. Under direct exposure to the solar rays, how- 

 ever, this acid gas is again decomposed, and its oxy- 

 gen is restored to the atmosphere; while, in the shade, 

 no such operation takes place, and the air, therefore, 

 remains permanently depraved. 



" But farther, it also appears, that the produc- 

 tion of oxygen is entirely confined to the leaves and 

 other green parts of plants ; and that the flowers, the 

 fruits, the stems and roots of vegetables, both in 

 sunshine and in the shade, convert always the oxygen 

 gas of the air into carbonic acid." 



It has been a favourite opinion with many, and Utility of 

 particularly with M. Theodore De Saussure, that carbonic 

 carbonic acid gas is necessary for healthy vegetation ; ac 'd. 

 and many experiments have been made with a view 

 to determine it. Mr Ellis has analysed the reason- 

 ings deduced from those instituted by M. De Satis- 

 sure with great address, and has given a most am- 

 ple confutation of the inferences made by that gen- 

 tleman from his own premises ; and further, he has 

 detailed some experiments made by himself, which 

 corroborate his opinions. We regret that we can- 

 not take even a rapid survey of the ingenious reason- 

 ings employed by Mr Ellis : the general conclusion 

 however is, that vegetation owes its perfection to 

 oxygen gas, and not to carbonic acid gas; that, on jj_ a (f or j 

 the contrary, this acid is injurious to vegetable life, iugnxyge 

 unless decomposed " by the agency of solar light." 

 In that case, indeed, the oxygen produced from the 

 decomposed carbonic acid is beneficial, so that vege- 

 tation will go on in a pure atmosphere, containing 

 not more than riVo P art f carbonic acid. 



Leaves have been supposed ta possess the property Detompo 

 of decomposing water as well as caibomc acid, when Mtion of 

 exposed to the light ot the sun ; and Berthollct has water. 



