18 
all have thick leaves and this fact is the more conspicuous if other species 
in their genera are used for comparison, rather than if other formations 
in their entirety are brought into consideration. Thus the two D.avallice 
on the strand average 0.24 millimeter in thickness; D. pallida and 
D. decurrens, of the high forest, average only 0.13 millimeter. If 
Antropliyum were represented here it would raise the average. A. 
obtusum is a strand fern along Illana Bay. 7 Whether thick or compara- 
tively thin, all strand ferns here are coriaceous in texture ; all except 
the Nipliobolus are glabrous, and the latter is notably glabrous in its 
genus; the woolly covering of some of our Nipliobolus species would be 
fatal where salt spray blows. All have a waxy-shining epidermis. All 
except one have a hypodermis lacking chlorophyll ; this serves to mitigate 
the insolation more often than it operates as a reservoir of available 
water. 
THE SALT MARSH. 
The habitat of A chrostichum is a part of what Schimper calls the Nipa 
formation, which Whitford, to show how far it is from being a unit, 
calls the Nipa- Acanthus formation. Achrostichum never grows with 
luxuriant Nipa, but does with 'Acanthus and in marshes still fresher than 
those which are characterized by this plant, its landward boundary being 
the extreme limit of saltiness at any tide. Since Acanthus has a rather 
narrower landward range and is not pantropic, as is Achrostichum, this 
fresh-brackish marsh, where Nipa does not grow, might well be named 
after Achrostichum. 
Achrostichum is structurally so perfectly adapted to its part arid, 
part watery habitat that, in spite of its occasional salty substratum and its 
exposure to the sun and at times strong winds, the fronds -reach to a 
height of 1 meter to 1.8 meters, or more. The pinnae are thick, coria- 
ceous and entire, with thick epidermal walls and hyaline epidermis, 
underlain above by two layers of irregular hyaline hypodermis and below 
by one incomplete one. There are two layers of compact, palisade-like 
parenchyma. So far, the structure is xerophytic, which seems to be an 
adaptation to the insolation rather than to any saltiness of the substratum, 
since the hypodermis is rather thick-walled. But a large, stout fern with 
very vigorous neighbors must not have xerophytic structures which inter- 
fere with active photosynthesis. The stomata of Achrostichum are .very 
numerous and rather large, occupying 35 per cent of the whole nether 
surface, a greater proportion than in any other fern of our flora. The 
air which surrounds this fern, though sometimes drying, is more often 
humid and a wet nether surface would long remain so. As a protection 
against the closure of the stomata by water, the margin, 0.33 millimeter 
broad, -is thin, cartilaginous, hyaline, and deflexed ; and the young fruit- 
ing surface is effectively covered by large paraphyses with branched, oily 
7 Drymoglossum piloselloides, a thick-leaved relative of Niphobolus, is a strand- 
epiphyte in some places in the Philippines. 
