622 CONSERVATIVE ORGANS. [LECT. XI. 



from being membranous and from the nature 

 of their attachments, are moveable in the air, and 

 thence have constantly a fresh atmosphere of that 

 fluid applied to their breathing apertures; this 

 mobility of the leaf supplying, in some degree, 

 the motion of the thorax and the diaphragm in the 

 more perfect animals. The plants which have very 

 thick and immoveable leaves, on the contrary, or 

 which are devoid, 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 consequently, as we have al- 

 ready seen, are less amply supplied with breathing 

 apertures. I have not been able to ascertain whe- 

 ther the apertures themselves have the power of 

 opening and shutting; but from the appearance 

 of the orifices of these organs, as they are seen on 

 the leaf of Indian Corn, when very highly magnified 

 (see Plate 10, fig. 15), it is not improbable that 

 some degree of dilatation and" contraction takes 

 place, although we cannot determine the fact. In 

 structure these organs seem well adapted for the 

 purposes of vegetable respiration, when we con- 

 sider 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 description. 

 The air is admitted through the funnel-shaped 

 pore, whiqh perforates the cutis, into a vesicle si- 



