— io5— 



long as the nether surface is not wet, or is imperfectly 

 so, the surface tension of a drop would cause it to be- 

 come spherical on. an ,edge as sharp as the one under 

 discussion, and it would therefore fall off. 



Overfullness of the margin causes an effect like that 

 produced by convexity of frond. Such margins are 

 wavy or crisped, alternately raised and depressed. Water 

 will, of course, run to the margin where it is lowest, and 

 only to this point; these are the places where it would 

 have to run upward to wet the nether surface. An ex- 

 ample is Polypodium macrophyllum. Such fronds can 

 hardly be torn, because the extra length of their margins, 

 allows them merely to straighten if the fronds are bent 

 toward the other side. A ciliate margin is, as a rule, an 

 obstacle to the passage of water; for if the hairs are not 

 wet, a drop must pass over their ends, from which it 

 will inevitably fall off, but if they are wet, they usually 

 furnish an opportunity for water to run down far enough 

 to fall, instead of allowing it to pass to the nether sur- 

 face. As a matter of fact, neither the hairs nor the 

 cuticle of plants in general are very readily wet. 



If we suppose a frond to be horizontal, then the pos- 

 sibility of a drop passing from the upper to the nether 

 surface depends upon the area of contact which it can 

 preserve with the frond while rounding the margin. If 

 the frond is in some other position, the area of contact 

 is still a very important factor. Unless this area is suf- 

 ficient to allow a drop to flatten into a broad enough oval 

 markedly to reduce its relative surface over which it 

 would be were the drop a sphere, and thus to overcome 

 the force with which gravity can act to remove the drop 

 from the leaf, it will inevitably assume the form of a 

 sphere, and fall. 



We have just seen that the surface tension of a drop 

 must prevent it passing around a sharp edge ; now, if 

 drops run to a toothed margin they must run in part 

 over the sinuses. If the sinus were a curve with a ra^ 



