— loy — 



or not at all so, in Nephr odium diversilobum and Steno- 

 semia. In Polypodium celebicum and a number of its 

 congeners the frond is cut so closely to a very hairy 

 rachis that the hairs on the latter may obstruct the 

 sinuses. 



I am convinced that here we have the correct inter- 

 pretation of serrate and otherwise inoffensively toothed 

 margins. These structural peculiarities are among those 

 most widespread, but hitherto they have been accepted 

 as facts without an attempt at a general explanation 

 being made. There are doubtless a sufficient number of 

 instances in which this .explanation will not apply, and 

 many more in which its application is not evident, but 

 among these must not be included the cause of the exist- 

 ence of those spiny margins, properly regarded as xero- 

 phytic characters, which are produced directly by strong 

 insolation, dryness or exposure to wind, for these more 

 often between the sharp teeth have rounded sinuses 

 through which water might run more readily than it 

 would around a straight or slightly convex margin. 



Dryness of the nether surface in the ferns is doubly 

 desirable, both for the sake of the fructification and also 

 to avoid plugging the stomata. The protection of the 

 fruit will be discussed later. The stomata are confined 

 to the nether surface of every vegetative frond in the 

 entire fern flora of San Ramon except in the single case 

 of Monogramma, which has no differentiated ventral sur- 

 face. Epiphytes dispense with incised margins more 

 commonly than do terrestrial forms because, if their 

 nether surface should be wet the water will be removed 

 with relative promptness by evaporation; but among 

 both groups, epiphytes and geophytes, it is almost invari- 

 ably true that fronds which have margins entire in de- 

 tail have other devices adapted for keeping their nether 

 surfaces dry. Among these are sharp, deflexed or ruf- 

 fled margins, convex or minute ultimate divisions of 

 other forms adapted to the facile escape of water from 

 the frond. 



