98 SYLVA FLORTFERA. 



dew, which, falling in the night, gathers on 

 certain trees ; but these dews melt in the sun, 

 whereas manna whitens and hardens in it. 



The evaporation of leaves, says Decandolle, 

 is one of the most obvious and important of 

 their functions. No person can deny it who 

 has noticed the drops of clear moisture on 

 the points of leaves, even in hot-houses, where 

 they cannot be affected by the dew ; or who 

 has traced the movement of a mist in a 

 still evening, as it raises itself from fields 

 planted with vegetables ; or who has seen the 

 rising of clouds from forests, and the ascent 

 of vapoury columns from the same place be- 

 fore the formation of a storm. In fact, plants 

 lose, by evaporation from their leaves, the 

 greatest part of the moisture which they take 

 in by their roots. The organs which are 

 chiefly employed in evaporation are the slits, 

 and also the hairs, which latter organs are 

 therefore more abundant in young shoots, and 

 in those parts whose evaporation is most ac- 

 tive. The sudden and powerful operation of 

 the sunbeams, after a passing drizzling rain, 

 favours not unfrequently the perspiration of 

 oxidized slime, and of sweet drops, which are 

 known by the name of honey dew ; the lime 

 and sycamore usually have a great deal of it 

 on their leaves in the heat of summer, and 



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