312 ANATOMY OP LEAVES. 



form the respiratory function. But the most perfect 

 plants are furnished with leaves, which, being mem- 

 braneous and peculiarly attached, are moveable in the 

 air, where a perpetual supply of that fluid is constant- 

 ly presented to their breathing apertures ; this mobili- 

 ty of the leaf supplying, in some degree, the motion 

 of the thorax and the diaphragm in the more perfect 

 ani mils. The plants which have very thick and im- 

 moveable leaves, on the contrary, or which are de- 

 void of leaves, as they resemble the cold-blooded and 

 slow-moving animals in their tenacity of life, like them, 

 also, require a smaller supply of air, and consequent- 

 ly, as we have already seen, are less amply supplied 

 with breathing apertures. In structure these organs 

 seem well adapted for the purposes of vegetable res- 

 piration, when we consider that the changes effected 

 by this function in the sap of vegetables in the leaf are 

 not required to be so quickly produced as those in the 

 blood of animals ; even of insects of the lowest de- 

 scription. The air is admitted through the funnel- 

 shaped pore, which perforates the cutis, into a vesicle 

 situated under it ; and which probably communicates 

 with the cuticular cells, as these are, in general, found 

 filled with air. The aqueous contents of the cells 

 that form the parenchyma of the leaf, are thus brought 

 into immediate contact with the atmosphere. It is 

 not easy to assign a reason why these apertures are 

 found on the under disk only of the leaves of trees, 

 while they appear on both disks of herbaceous leaves ; 

 there being lymphatics on both disks in the former as 

 well as of the latter description of leaves. If any con- 

 nexion could be traced between the returning vessels 

 and the apertures, the difficulty would be diminished, 

 the situation of these vessels being on the lower disk 

 of the leaves of trees. 



With regard to the origin of these apertures, Saus- 



