158 TRANSPIRATION. 



these are more frequent on the lower sides of the 

 leaves. If we dip these leaves in water, the wet 

 collects in the form of drops, which roll off when 

 the leaf is shaken and leave no moisture behind. 

 It might be thought that the under side of a leaf 

 would be protected by its position from rain, but 

 if we turn over a leaf on a dewy morning, we 

 shall find that the water is as much on its under 

 as its upper side. We forget, when we speak of 

 the falling of the dew, that this is only a figure of 

 speech, that the dew is really condensed just as 

 much on the lower as on the upper side of a leaf. 



These waxy coverings to the leaves and their 

 garments of hair prevent excessive evaporation, 

 and are often also a defence against the attacks 

 of animals. 



Another means of defence is the occurring of 

 numberless little projections all over the surface 

 of the leaf. When the falling drops of water roll 

 over such a surface, they cannot dislodge the air 

 which fills the depressions. As the stomata are 

 always in these pits, they are kept dry, and are 

 not wet even if the whole plant is immersed in 

 water. This means of protection is common with 

 plants growing in places where they are liable to 

 be immersed for weeks together, as with some 



