310 ANATOMY OF LEAVES. 



From what we are able to learn concerning the 

 structure of these pores, there is reason to believe that 

 they are the respiratory organs of plants ; though 

 phytologists have considered them as intended for the 

 functions of absorption and exhalation. The idea 

 that they are absorbing organs, is supposed to gain 

 support from the circumstance, that leaves absorb 

 more powerfully with their lower than with their up- 

 per surface : but although leaves absorb chiefly by 

 their inferior surface, yet, this does not prove that 

 these apertures are the absorbing organs ; for we find 

 none of them on the lower side of the leaves of JYym- 

 phcea (the Water Lily) and other aquatics, which have 

 floating leaves ; although these leaves absorb power- 

 fully by their lower surface, and exhale by their up- 

 per, which is covered by these apertures. It is still 

 more difficult to accord with the opinion, that the 

 same foliar apertures perform such opposite functions 

 as those of absorption and exhalation ; although there 

 is nothing incongruous in supposing that they are both 

 exhalent 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 De- 

 candolle proved that the aqueous transpiration is great- 

 est in those plants which are supplied with the greatest 

 number of apertures. To prove that these apertures 

 are the foliar exhalents, and that no exhalation takes 

 place when they are obstructed, Mr. Thompson 

 made the following experiment with two twigs ofLau- 

 rustine, each having 4 leaves nearly of the same size, 

 and brought to the same weight in opposite scales. 

 The lower disks of all the leaves on one twig were 

 next brushed over with a composition of mucilage of 



