180 On the Physiologj/ of Vegetalles. 



forniinu; oxvgen bv tho decomposition of water, is no reason ot 

 all tor such an appellation ; and it is certain that the leaves 

 contain vastly lcs,s air in their interior, than any other part ot 

 tlie plant, and are not therefore a vehicle for air. But though 

 they have no air within, tliev still merit the name they have ac- 

 quired, bv being the constant cause of the motion of air at the 

 exterior; so that the vcrv oxygen the leaves give out would pro- 

 bably remain alviost stationary , on account of its weight, under 

 the trees, rather than circulate around, were it not for the in- 

 numerable little fans that by their incessant motion produce an 

 excessive circulation, which is rarely stopped, and is most violent 

 in the lowest spots, and where the general stagnation of air is 

 most likely to exist. But the motion of the leaves not only 

 changes the gases (which descend from the higher regions by 

 the help of the currents of air), but increases also the natural 

 evaporation of the leaves. How exquisitely beautiful is then the 

 arrangement of the lungs of the plant, when they are considered 

 asset in motion by the spiral wire or muscles of the plant, in or- 

 der to disperse the oxygen ! and that motion exactly propor- 

 tioned by Nature to the situation of the ground, and the neces- 

 sities of the sort of country in which they are placed ! On the 

 high hills of Scotland or Sweden, where no putrid air is disco- 

 verable, the^ri grow, which give but little oxygen; for they have 

 no swamps to rectify, no animal breathing to purify: the na- 

 tural motion of the air is therefore exercise enough for them, 

 and to disperse their pollen. Hence the firs have no spiral wire 

 in their leaves, and in their leaves no motion, and fewer muscles 

 (except in their wood) than any other plant. But behold the con- 

 trast ; — The low and swampy grounds loaded with aquatic plants 

 and trees, where constant motion is necessary to the purifying 

 of the air — Here Nature not only bestows a quantity of oxygen 

 (which its trees emit continually), but she has loaded the 

 peduncle of the leaf with a cpiantity of spiral wire, which keeps 

 its leaves in perpetual motion. View only the alele, or the black 

 poplar. It is not because its leaf-stalk is broad one way and 

 thin the other (see fig. 14), that the leaf is for ever moving: this 

 shape would cause its constant action, when the ivids part of 

 the leaf was y(7f.'//o' the wind. But what (but the muscle) could 

 keep it constantly in that position P The muscles alone could 

 do this, by contractii g and dilating it, according to the dryness 

 or moisture of the wind that blows. It is by this means the 

 leaf-stems of all the poplars are a trifle more or less turned to the 

 wind, and in this position they will ever be found. Those plants 

 have most spiral wire which grow in swampy grounds ; those 

 trees have most motion that are buried in low valleys, where 



borti 



