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bules, pyramids, filaments, separate the more aqueous, 

 or gross parts of the juice, that rises from the root. 

 The air contained in the trachae of the stalk and branches, 

 by dilating itself more and more, presses the ligneous 

 fibres, and by that means accelerates the course of the 

 sap, at the same time that it causes it to penetrate into 

 the neighbouring parts. 



When night approaches, the inferior surface of the 

 leaves begins to perform one of its principal functions : 

 the little mouths it is provided with open themselves, 

 and receive the vapours that float in the atmosphere. 

 The air of the trachae is confined within them ; their 

 diameter is lessened; the ligneous fibres being less 

 pressed, enlarge themselves, and admit the juices con- 

 veyed to them from the leaves: these juices join them- 

 selves to the residue of that which had arisen in the 

 day-time, and the whole mass tends towards the roois. 



This seems to be exactly the mechanism to which the 

 motion of the sap may be reduced. You now see, 

 more clearly, the design of the direction of the leaves, 

 and of their admirable reverting ; the inferior surface 

 being intended for imbibing the dew, should face the 

 earth, from whence this vapour rises gradually at sun- 

 set. But when I say that the principal office of this 

 surface, at least in trees and shrubs, is to receive the 

 dew, I would not infer that the opposite surface is inca- 

 pable of it ; that may, perhaps, absorb vapours that are 

 more rare. 



Experiments that are well made, seem to prove that 

 the inferior surface of the leaves or trees serve likewise 

 for insensible perspiration. Those leaves, in which this 

 surface was endued with a matter impenetrable by 

 water, drew in and transpired much less, in an equal 

 time, and with the same management, than leaves of the 

 rjame size and likeness, whose inferior surface had not 

 been endued with such a varnish. It seems to have re- 

 sulted from the same experiments, that there is but 

 little perspiration by the upper surface ; we may thence 

 infer, that one of its principal functions is, to serve for a 

 helter or defence to the lower surface : and that, no 



