618 CONSERVATIVE ORGANS. [LECT. XI. 



have floating leaves ; although these leaves ab- 

 sorb powerfully by their lower surface, and exhale 

 by their upper, which is covered with these aper- 

 tures. Decandolle and Sprengel's remarks regard- 

 ing succulent and fleshy leaves, are also incor- 

 rect ; for, in the genus Aloe, which is supported 

 chiefly by absorption, the apertures are compara- 

 tively few ; and the function of absorption in these 

 plants seems to be connected with a small papilla, 

 which rises in the centre of every mesh ; and pro- 

 bably acts in the same manner as radicles on roots. 

 It is still more difficult to accord with the opinion 

 that the same foliar apertures perform such oppo- 

 site functions as those of absorption and exhalation ; 

 although there is nothing incongruous in supposing 

 that they are both exhalant and respiratory organs. 

 That they exhale, was first rendered probable by the 

 experiments of Treviranus, who found that plates 

 of glass applied to the lower disks of leaves were 

 soon covered with drops of water, while they were 

 not at all bedewed when they were affixed to the 

 upper disks ; and Decandolle afterwards proved 

 that the aqueous transpiration is greatest in those 

 plants which are supplied with the greatest num- 

 ber of apertures. The opinion, however, still 

 prevailed, that leaves exhale also by the upper 

 disk, although the majority of leaves have no 

 apertures on that disk; but before yielding im- 

 plicit credence to the assertion, that these aper- 



