ANATOMY OF PLANTS 203 



to the height of an inch, the most juicy plants are often 

 found to grow to an astonishing height. They can only be 

 nourished by means of their green surfaces. In hot-houses, 

 too, we never attain a brisk growth so much by watering the 

 roots of the plants, as by an artificial wetting and sprinkling 

 of the plants from above. Evident as all this is, it is still 

 a difficult matter to explain this absorption, upon com- 

 mon principles, through the closed sides of the cells. We 

 might indeed ascribe this effect to the under surface of the 

 leaves, on which principally the slits are seen; but as dew 

 and rain much more frequently fall than ascend, we cannot 

 avoid confining this absorption of the vapours and fluid drops, 

 to the upper surface, on which supposition, we are again 

 forced to betake ourselves to an organic perspiration. 



S20. 



The evaporation of leaves 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 places before 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 proportion of the water absorbed, to that 

 lost by evaporation, is as 15 to 13, seldom as 4 to 1. It is 

 hence that a branch without leaves, when it has been placed 

 in water, becomes heavier than one in a state of frondescence, 

 because it wants the organs through which it may relieve it- 

 self of its superfluous nourishment. 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 

 active. 



