BOTANY. 



Vegetable 

 Phy.iolugy. 



Purifica- 

 tion of the 

 atmosphere 

 by vege- 

 table!. 



Coloura- 

 tion of 

 p!ajit. 



inferred, that a portion of the apparent oxygen it 

 derived frcin the water, while the hydrogen, with 

 the carbon (of the decomposed carbonic acici), forms 

 the inflammable parts of the plant. This hypothesis 

 is ingenious, and has received some support from 

 Saussure's experiments. But many objections may 

 be urged against it ; so that, without farther proof of 

 its correctness than we at present have, we must 

 unite with Mr Ellis in rejecting it. 



We have in a former part hinted at the supposed 

 purification of the atmosphere by means of plants. 

 The beauty of the idea has led many to adopt it, 

 without investigating its truth ; and Mr Ellis has 

 here, too, thrown down the gauntlet ; he asserts, 

 that the means supposed to effect such great ends 

 are wholly inadequate, and that although there are 

 certainly some means by which the air is freed of the 

 noxious ingredients derived from vegetation, germi- 

 nation, and respiration, yet the present state of che- 

 mical science does not permit us to say what they 

 are. Mr Ellis meets, with energy, the cavils of 

 those who may urge the doctrine of this purifying 

 process as necessary in the economy of nature, and 

 as illustrative of the wisdom of Providence. " It 

 behoves us," says he, " to employ no ordinary por- 

 tion of delicacy and caution in pronouncing on the 

 general plans and purposes of Providence, from the 

 little and partial views of nature, which at present 

 we are permitted to take, lest, in the effervescence 

 of our zeal, we degrade the wisdom we pretend to 

 exalt, and pervert the designs of the goodness we 

 profess to revere." 



The colouration of plants presents one of the most 

 interesting, and, at the same time, obscure branches 

 of physiological research. Humboldt attributes the 

 green colours of leaves to the agency of hydro- 

 gen, because he had observed some plants retain 

 their green colour in mines. Saussure, however, 

 could not increase the green of plants by means of 

 hydrogen. Humboldt also ascribed the white co- 

 lour to oxygen, which seems to be erroneous, as this 

 oxygen existt'd in a state of combination previous 

 to its being made apparent, and cannot therefore 

 be proved to produce this white colour. Scnebier's 

 phlogistic hypothesis is unworthy of detailed no- 

 tice. His subsequent opinions respecting the ope- 

 ration of carbon, do not seem to be better founded. 

 This philosopher, as well as Berthollet, determined 

 many important facts. There is a very evident con- 

 nexion between the decomposition of carbonic acid 

 gas and the green colour of leaves, as is shewn by 

 the following results. Green leaves only yield oxygen 

 gas ; the parenchymatous substance of the leaf is the 

 cat of decomposition of the gas, and of the green 

 colour } the colouration will take place in leaves se- 

 parated from the stem, so that there can be no living 

 action in them ; consequently the colouration is inde- 

 pendent of the life of the plant. 



It appears from the various experiments of Ber- 

 thollft, Senebier, and others, that solutions of the 

 colourable parts of leaves are rendered red or green 

 according to the predominance of acid or alkaline 

 matter; hut similar effects will be also induced on 

 entire leaves. Etiolated leaves, and those reddened 

 ky age, pass into green iu alkaline fluids ; those that 



are yellow from decomposition become brown in the Vegetal 

 9ime circumstances, li'lis observed, that if the green fnysioli 

 colour had been previously affected by the action of *" "*" v ~" 

 water, that alkalies did not restore it. On these 

 facts this physiologist ventures to presume, " that 

 these same agents (acids and alkalies) if present, 

 will exert a similar action on leaves during their 

 growth." 



He first establishes the existence of a large quan- 

 tity of alkaline matter in the leaves, and then shews 

 that the separation of the carbonic acid, and its sub- 

 sequent decomposition, render the alkali predomi- 

 nant, and consequently better fitted to exert its spe- 

 cific influence " on the colourable juices of the plant,"" 

 which produces the green colour of the leaf. So 

 that, according to Mr Ellis's views, the decomposi- 

 tion of the carbonic acid answers two purposes, the 

 production of oxygen gas and the formation of the 

 green colour. 



If the green colour depend on the predominancy of 

 alkali, it may be reasonably supposed that the white 

 colour depends on a deficiency of it. Senebier's ex- 

 periments support this supposition, and his results 

 have been confirmed by Davy and others. This 

 whiteness of colour will continue so long as the acid 

 abounds ; and accordingly we lind, that plants grow- 

 ing in the shade are etiolated, or devoid of the green 

 hue, until they are placed in the full sunshine. The 

 various shades of colour exhibited by different leaves 

 immediately before their fall are affected by similar 

 causes, and consequently are explicable on the same 

 principles. In the same way, too, the variegation of 

 different leaves may be resolved, or at least the agents 

 that produced them may be pointed out. The mode 

 in which these effects are induced, Mr Ellis does not 

 attempt to detail, being satisfied with the general 

 fact. It would open a field of interesting inquiry, 

 to examine, with care, the gradual changes from 

 green to white, with all the intermediate and colla- 

 teral varieties of colour; and, it is probable, that in 

 the course of such researches, many valuable facts 

 would develope themselves, and thus tend more clear- 

 ly to elucidate this branch of vegetable physiology. 



The necessary agency of light in promoting the fjhemit 

 colouration of plants, has been made sufficiently evi- agency, 

 dent in the preceding detail ; but it yet remains to 

 be determined in what manner it exerts this agency'; 

 a question of much delicacy and difficulty. Adopt- 

 ing Mr Davy's notions respecting the nature of 

 electrical agency, Mr Ellis has instituted an ana- 

 logy between the chemical agencies of this fluid and 

 of the solar ray, supposing that the action of ca- 

 lorific rays corresponds with that of the positive 

 electricity, and that that of the deoxydizing rayg 

 correspond with that of the negative electrici- 

 ty ; the former promoting combination, and the 

 latter decomposition. Atter a full examinatio 

 of the facts and opinions that are maintained on 

 the subject of colouration, the following are his 

 general conclusions. " We may observe, in the 

 first place, that, by the chemical agency of this 

 subtile matter, the saline compounds of plants 

 are decomposed, and the acid and alkaline mat- 

 ter, thus devt loped, combine with the colourable 

 juices of the vegetable. In consequence of this coo 



