502 Plant Case for growing Plants 



through the whole plant; but the exclusion of light from any 

 part of the plant affects that part only ; and even the total ex- 

 clusion of that agent only deprives the plant of certain properties 

 necessary 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 sun- 

 shine, are so manifest, both in their nature and effects, as to jus- 

 tify the ascription of a name to the latter process distinct from 

 that given to the former. It might, perhaps, be denominated the 

 chemical process, in contradistinction to that named physiological. 

 It would contribute much, we think, to simplify our en- 

 quiries concerning vegetation, to bear in mind these distinc- 

 tions : to consider the one process as accomplished by the 

 agency of the air, and essential to the life and growth of the 

 plant; the other, as subordinate, depending on the agency of 

 light, and though necessary to the perfection of vegetation, yet 

 not essential to its existence. In this manner, each process may 

 be followed out separately, both in regard to its immediate 

 effects and remoter consequences, without clashing with the 

 other; and the apparently discordant and even contradictory 

 phenomena which, on a first view, they seem to exhibit, may be 

 reconciled, and considered, not less in theory than in fact, as 

 conspiring together to form one harmonious and perfect whole. 



Applying these views to the subject under consideration, we 

 see no difficulty in comprehending how the same identical vo- 

 lume 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 experiments 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 cir- 

 cumstances, in the glass cases of Mr. Ward, that is, when their 

 plants are similarly exposed to vegetate alternately in sunshine 

 and in shade. And as the former 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 continuing 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 deterior- 

 ation of the air, occasioned by vegetable growth, is counteracted 

 by another process, necessary to the perfection of the plant ; 

 and, amidst the vicissitudes of perpetual change, the atmosphere 

 of these cases is maintained in a state of nearly uniform composi- 



