THE VEGETABLE KINGDOM. 387 



rivers, do not present this organization. Having no con- 

 nection with the atmosphere, these aerial cavities would be 

 of no service; hence they display quite a peculiar arrange- 

 ment, approaching more that exhibited by fish, which have 

 a special respiratory apparatus, the branchiae, so disposed 

 as to take up imperceptible portions of air contained in the 

 water in large enough quantity to suffice for their respira- 

 tion. An analogous disposition is seen in certain plants of 

 the family of the naiads, Nai'adeaa, 1 which live in our pools 

 and ditches. Their leaves are unprovided with epidermis, 

 and represent a kind of branchiae, constructed to act upon 

 the atmospheric particles contained in the medium in which 

 they live. Of this class are the potamogetons, or pond- 

 weeds, which, in respect to their respiration, considered in 

 an isolated point of view, are in reality fish-plants. 



The respiration of animals is injurious to the composition 

 of the atmospheric air ; they vitiate it incessantly, either by 

 absorbing the vital principle oxygen, or by diffusing in it 

 a deadly poison carbonic acid. 



It has been calculated that the human species alone con- 

 sumes annually 160,000 millions of cubic yards of oxygen, 

 and that animals quadruple this amount. 



On the other hand, every man exhales daily about 8 oz. 

 troy of carbonic acid gas, which gives about 2i oz. troy of 

 combustible carbon. Hence, without reckoning the amount 

 produced by animals, the quantity of carbon poured into 

 the atmosphere by the population of France alone may be 

 computed at about 2,500,000 tons per annum. 



This alarming alteration in our atmosphere is enough to 



1 Also called Fluviales, a small natural order of endogens. TR. 



