1 68 LOSS OP FLUID FROM T11E LEAVES. 



under favourable circumstances, from the surface of the Leaves, 

 as from the pores of the skin of a Man. 



Exhalation. 



253. If a glass vessel be placed with its mouth downwards, 

 on the surface of a meadow or grass-plot, during a sunny after- 

 noon in summer, it will speedily be rendered dim in the interior, 

 by the watery vapour which will rise into it ; and this will soon 

 accumulate to such a degree, as to run down in drops. From 

 an experiment of this kind, repeated by Bishop Watson during 

 several successive days, on a meadow which had been cut during 

 a very intense heat of th.e sun, and after several weeks had been 

 passed without rain, it was calculated by him, that an acre of 

 grass-land transpires in 24 hours not less than 6400 quarts of 

 water. This is probably an exaggerated statement ; as the 

 Bishop does not seem to have been aware how completely tran- 

 spiration is checked during the night ; but it will serve to give 

 an idea of the enormous amount of fluid, which must be thus 

 disengaged. Any person walking in a meadow on which the 

 sun is shining powerfully, especially in a hot day in summer, 

 when the grass has not long previously been refreshed by rain, 

 may observe a tremulous motion in distant objects, occasioned 

 by the rising of the watery vapour; exactly resembling that 

 which takes place along the sea-shore, when the sun shines 

 strongly on the pebbles that have been left in a moistened state 

 by the retreating tide. 



254. It is necessary, however, to distinguish the evaporation 

 which is the cause of the latter occurrence, from the peculiar 

 function we are now considering ; which, as we shall see, is in- 

 fluenced by circumstances that only act during the life of the 

 Plant, in such a manner, as to prove it to be something of a 

 different character, from that which we observe in dead sub- 

 stances. All moist bodies exposed to a tolerably warm and dry 

 atmosphere have a tendency to become dry, the fluid they con- 

 tain, slowly passing off in the form of vapour. The rapidity 

 with which this takes place, will depend upon the amount of 

 heat to which they are exposed, and upon the degree of dry ness 



