LECT. XI.] ANATOMY OF LEAVES. 619 



tures are the foliar exhalants, it was necessary to 

 prove that no exhalation takes place when they 

 are obstructed; and to determine this point I 

 made the following experiment: Two twigs of 

 Laurustine, each having four leaves nearly of the 

 same size, were cut from the plant, and brought to 

 the same weight by being placed in opposite scales. 

 The lower disks of all the leaves on one twig were 

 next brushed over with a composition of mucilage 

 of gum arable and a small proportion of Traga- 

 canth; and when this was dry, each twig was placed 

 under a cylindrical glass jar containing air, and 

 immersed in a saucer of water. In a short time the 

 sides of the jar containing the twig in its natural 

 state, were covered with drops of water ; but, at the 

 end of two days, not the smallest quantity of mois- 

 ture appeared on the sides of the jar containing the 

 twig, the apertures of the leaves of which had been 

 obstructed by the mucilage. The conclusion to be 

 drawn from this experiment is, that that surface 

 only on which apertures exist exhales, and conse- 

 quently that these apertures are the exhaling or- 

 gans. This experiment leads us, also, to draw a 

 conclusion from the experiments of M. Bonnet, dif- 

 ferent from that drawn from them by that author. 

 He concluded that, because the leaves of trees 

 which were laid with their lower disk upon water 

 remained longer green and fresh than those which 

 had the upper disk applied to the water, this fluid 



