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dius equal to or greater than that of the drop, or if the 

 radius were not much less, the drop would have an area 

 of contact greater than it would have in passing the 

 entire margin, and so it would pass more readily. Such 

 sinuses are found in Dipteris, but they are so placed that 

 water to reach them would have to run up-hill. I have 

 found such sinuses in no other fern. If a drop runs to 

 a sharp sinus, or to one much narrower than the diam- 

 eter of the drop, then the sides of the sinus will hold it 

 up and make it run outward to where the sinus widens 

 or the ends of the teeth are reached. If the sinus widens 

 sufficiently to allow the drop to run through, then even 

 though the frond is quiet enough so as not to cause it to 

 be shaken off when its contact is limited, and although 

 it might return to the body of the frond while still touch- 

 ing both sides of the sinus without its having to run up- 

 hill, nevertheless it is more than likely to leave the frond 

 because of its impetus in falling through the sinus ; this 

 condition is easily demonstrated. If a drop runs out onto 

 a tooth it loses its opportunity to pass to the nether sur- 

 face by diminishing its possible contact, just as it does 

 if it runs on to a caudate tip. In general, water must 

 run to the teeth rather than to the sinuses, because its 

 surface tension prevents its starting over an edge if it 

 can run along it. Thus a toothed or cut margin in all 

 parts of its periphery is provided with a water-removing 

 structure such as caudate leaves have at the apex. I have 

 demonstrated the inability of water to pass around a 

 narrowly or sharply cut margin by experiments on vari- 

 ous ferns. A very much larger proportion of terrestrial 

 than of epiphytic ferns have such margins ; as epiphytes, 

 by virtue of their position, will be injured for a compara- 

 tively short time by their wetness of the nether surface. 



In a few ferns there are special adaptations to make 

 the sinuses impassable to water. The sinus is elevated 

 in Nefhrodium syrmaticum and others. It is obstructed 

 by local hairs, the rest of the margin being less ciliate, 



