PLANT CASE FOR GROWING PLANTS. 



«85 



inner bark, whereby it may be rendered fit for 

 the purposes of nutrition and growth. In its nature 

 and object, therefore, as well as in the specific 

 change which it produces in the air, this pro- 

 cess closely resembles the function of respira- 

 tion in animals, and may thus with propriety be 

 deemed a physiological process. 



The second, or purifying, process, in which 

 oxyen gas is evolved, differs, in all respects, from 

 that which has just been described. It is, in a 

 great measure, independent of temperature ; at 

 least it proceeds in temperatures too low to sup- 

 port vegetation, provided light be present, an 

 agent not required for germination, nor essen- 

 tial to vegetable development. The organs by 

 which this process acts on the air are, as before, 

 the leaves ; not, however, by changing the qua- 

 lities of the sap in the vessels of those organs, 

 but by producing changes in the chromnle, or 

 colourable matter, in their cells, fo which it 

 imparts colour and other active properties. In 

 doing this, it does not convert the oxygen gas of 

 the air into carbonic acid; but, by decomposing 

 that acid gas, restores to the air the identical 

 portion of oxygen of which the former process 

 had deprived it. The former process, carried on 

 by the agency of the oxygen gas of the air, 

 was essential to living action, and affected the 

 (vell-being of the whole plant; that exercised 

 by the agency of light is not necessary to life; 

 is local, not general in its operation; and is cap- 

 able of proceeding in circumstances and under 

 conditions incompatible with living action. By 

 withdrawing the air altogether, or depriving it 

 of oxygen gas, vegetation soon ceases through 

 the whole plant; but the exclusion of light from 

 any part of the plant affects that part only; and 

 even the total exclusion of that agent only 

 deprives the plant of certain properties neces- 

 sary to its perfection, but not essential to its life. 

 These differences in the processes by which 

 oxygen gas is alternately consumed and evolved, 

 during the vegetation of plants in sunshine, are 

 so manifest, both in their nature and effects, as 

 to justify the ascription of a name to the latter 

 process distinct from that given to the former. 



Applying these views to the subject under 

 consideration, we see no difficulty in compre- 

 hending how the same identical volume of air 

 in the plant cases of Mr. Ward should, for so 

 long a period, serve the purposes of vegetation, 

 without becoming foul from within, or receiving 

 or requiring renewal from without. The expe- 

 riments of De Saussure furnish, as we have seen^ 

 examples of a similar kind; and supply, at the 

 same time, the desired explanation. The daily 

 depravation and subsequent purification which 

 the air underwent in the glass vessels of that 

 eminent chemist, must be equally accomplished, 

 under similar circumstances, in the glass cases of 

 Jlr. Ward, that is, when their plants are simi- 



larly exposed to vegetate alternately in sunshire 

 and in shade. And as the fonner found the air 

 to continue for many days together unchanged, 

 either in purity or in volume, when so treated ; 

 so must the air, in the plant cases of the latter, 

 preserve, under similar treatment, its original 

 composition and purity; not, however, by con- 

 tinuing always the same, but by simultaneously 

 undergoing opposite changes in sunshine, or 

 successive changes by alternate exposure to light 

 and shade, which mutually counterbalance each 

 other. Thus the deterioration of the air, occa- 

 sioned by vegetable growth, is counteracted by 

 another process, necessary to the perfection of 

 the plant; and, amidst the vicissitudes of per- 

 petual change, the atmosphere of these cases is 

 maintained in a state of nearly unifonn compo- 

 sition and purity. In this way, the same air by 

 changes of composition, like the same water by 

 changes in its state or condition, may be made 

 to serve over and over again the purposes of 

 vegetation. 



There is one circumstance of difference in the 

 experiments of De Saussure, as compared with 

 those of Mr. Ward, which it may be proper to 

 notice. In the experiments of the former no 

 soil was used, but only a thin stratum of watei-, 

 in which the roots of tlie plants were immersed, 

 covered the surface of the mercury, over which 

 the vessels were inverted. In the cases of Mr. 

 Ward, the plants were, set in earth. Now, 

 vegetable soil is known to deteriorate tlie air, by 

 fonning carbonic acid with its oxygen, in the 

 same manner as plants do; but the acid gas, 

 which may thus be produced, was found by De 

 Saussure to be decomposed by the joint agency 

 of tlie plants and light, like that produced by 

 ordinai-y vegetation; and, consequently, the air 

 suffered no permanent injury. Indeed, an ex- 

 cess of carbonic acid, not exceeding one-twelfth of 

 the atmosphere in which plants were confined, 

 accelerated their vegetation in sunshine, by in- 

 creasing the proportion of oxygen; whilst the 

 smallest doses of this gas proved injurious to that 

 process in the shade. 



The foregoing facts demonstrate the power of 

 light to decompose carbonic acid gas in plants. 

 This decomposition, however, can be effected 

 only by the concurring agency of the light and 

 the plant; and, whilst the acid gas is decom- 

 posed, the plant itself acquires a tint of green; 

 so that the evolution of oxygen gas by the plant, 

 and the fomiation of its green colour, always 

 proceed together. Now, as the chromule, which 

 imparts colour to the leaf, is lodged in the cells 

 of the parenchyme, it is in those cells that we 

 must suppose the decomposition of the acid gas 

 to be efl'ected, and jrom them also the oxygen 

 gas must proceed. The mode in which this 

 coloration is probably accomplished may receive 

 illustration from the facts which follow. The 



