36 
will be injured for a comparatively short time by wetness of the nether 
surface. 
In a few ferns there are special adaptations to make the sinuses 
impassable for water. _ The sinus is elevated in N eplir odium syrmaticum, 
Aspidium leuzeanum and Thayeria. It is obstructed by local hairs, the 
rest of the margin being less ciliate or nor at all so in Nephrodium 
diversilobum , S 'tenosemia aurita and 8. pinnata (fig. 2). In Poly podium 
celebicum and a number of its congeners the frond is cut so closely to a 
very hairy costa or rachis that the hairs on the latter may obstruct the 
sinuses. Single, fine projecting teeth or setae effectively obstruct the 
sinuses of N eplir odium syrmaticum, N. cyatheoides and A. leuzeanum. 
Other Philippine ferns with such teeth are Egenolfia appendicuilata and 
Leptochilus 'cuspidatus. 
I am convinced that we have here the correct interpretation of serrate 
and otherwise inoffensively toothed margins. These structural peculiar- 
ities 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 ex- 
planation will not apply and many more in which its application is not 
evident, but among these must not be included the cause of the existence 
of those spiny margins, properly regarded as xerophytie characters, which 
are produced directly by strong insolation, dryness or exposure to wind 14 
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 of 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 Mono- 
gramma , which has no differentiated ventral surface. Epiphytes dispense 
with incised margins more commonly than do terrestrial ferns 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 invariably true that fronds which have margins 
which are entire in detail have other devices adapted toward keeping 
their nether surfaces dry ; among these are sharp, deflexed or ruffled mar- 
gins, convex or minute ultimate divisions, or forms adapted to the facile 
escape of water from the whole frond. The general occurrence of other 
obvious devices intended to prevent water from reaching the nether 
surface of ferns which are without cut margins, together with their 
much more frequent absence on ferns which have cut margins, or in 
the latter event, together with their presence merely as supplementary 
14 Brenner 1. c. 
