352 Comparative Physiology. 



usually accompanied in him, as in other animals, with more or 

 less disturbance of various functions." The deficiency of 

 oxygen in rarefied air, from its want of condensation, causes 

 increased respiration to furnish the requisite quantity, which 

 augments the effects of a want of pressure. He takes no 

 notice of the pressure on plants, but the growth of plants in 

 rarefied air is feeble indeed, when compared with that under 

 the ordinary pressure of the atmosphere. Part is no doubt due 

 to the expansion by heat, and attraction of, and consolidation by, 

 light ; but the want of atmospherical pressure, and especially 

 of the motion caused by the pressure of atmospherical currents, 

 tends greatly to draw up plants in a sickly weak condition 

 in confined situations. Plants are never so vigorous as when 

 submitted to the free action of the air, under ordinary cir- 

 cumstances ; and plants grown under glass will always be found 

 more vigorous when suflftcient heat can be kept up to allow 

 of a free ingress of air. Alpine plants never thrive so well 

 in summer at the foot of a north wall, as at the foot of a 

 north hedge. The removal of injurious heat is indispens- 

 able and beneficial, but when accompanied by the sifting of 

 the wind through the hedge is still more beneficial. It is pro- 

 bable, though not hitherto allowed, that the respiration of 

 plants, or their emission of carbonic acid, which is a constant 

 function (as in animals), is due to the removal of waste or unsound 

 particles from the system, to which the motion by atmospherical 

 currents may be necessary. No vessels appropriate to inter- 

 stitial absorption have yet been discovered in plants ; but the 

 discovery of laticiferous vessels is only of late date, and in the 

 young growing shoots, where the removal of unsound particles 

 will be most needed, such vessels will be difficult to detect. 

 There is no muscular system in plants, as in animals, to produce 

 waste in a great degree; they cannot exist, however, without 

 inhaling oxygen, and, though part of the oxygen may be needed 

 in chemical decomposition, to reduce the starch, &c., into food, 

 yet the carbonic acid exhaled is likely to be, as in animals, from 

 waste. It would help to determine this, if, by experiment, ex- 

 posed plants were found to exhale more carbonic acid than jDro- 

 tected plants. It has been shown by the experiments of Bur- 

 nett (Journal of the Royal Institution, n. s. vol. i. ; Carpenter, 

 2d Con. p. 338.), that the evolution of carbonic acid in ve- 

 getables is a constant function, even when fixation is most 

 rapid in the full light of the sun. This seems a fact not gene- 

 rally known, as most other physiologists take no notice of it ; 

 Liebig says it is not the case. There seems little doubt, how- 

 ever, of its being correct ; and the inhalation of oxygen, and 

 exhalation of carbonic acid, would seem to infer a removal of 

 waste. 



( To he continued.) 



