TREE FERNS OF NORTH AMERICA—MAXON. 485 
ferences in venation, too, certain species having the veins and vein- 
lets entirely free, others having the midveins of the segments regularly 
united near their base by a short transverse veinlet; differences 
which have been largely overlooked in recent years, but which, 
especially in connection with the position of the sori, are of primary 
importance in the recognition of the species. Only a few species, of 
which H. horrida is one, have the stipes noticeably spiny. The 
scales of the rhizome and stipe also are unusual in their broadly oval 
to ovate form, blunt (or at least never long-attenuate) tips, and 
usually thin lax texture; and many of the species are nearly or quite 
devoid of any scales upon the under surface of the segments, probably 
because these are not needed as a protective covering. Indeed, in 
many respects this group, which is wholly tropical American, is one 
of unusual interest, although the recognition of the 20 North American 
species is by no means easy, largely because of their complicated 
taxonomic history. A somewhat detailed descriptive account of 
these has recently been published.* 
The lateral position and hoodlike form of the indusium common to 
the species of this section is shown in plate 12, which represents part 
of a segment of H. horrida at eight times natural size. The lax cob- 
webby covering of the veinlets here noticed may be observed in 
several other species in their young state; with age it disappears. 
THE GENUS ALSOPHILA. 
As Hemitelia differs from Eucyathea in having but “half an 
indusium,” so Alsophila departs still further in having no indusium 
whatever, or in a few species only a rudimentary or vestigial one, 
lying as a minute thin scale at the base of the receptacle, beneath 
the sporangia and concealed by them. Taking the place of indusia 
as a protective feature, however, there is noticed an increased develop- 
ment of paraphyses, variously shaped and colored, elongate, simple 
hairs, mixed among the sporangia and often exceeding them. Similar 
hairs are present inCyathea and Hemitelia, but are mostly shorter and 
not very obvious. 
The North American species of Alsophila, about 30 in number, are 
divided among the sections Eualsophila, Trichipteris, Lophosoria, 
and Amphidesmium (Metaxya). Eualsophila contains mostly arbo- 
rescent species similar to Cyathea in everything save sori; the others 
are represented within our area by a single species each. Of Eual- 
sophila may be mentioned the following more interesting or better- 
known species: Alsophila aspera, a common West Indian species, 
two pinne of which are shown in plate 7; A. elongata, a species rang- 
ing from Costa Rica to the Andes of South America, well marked 
by its huge, broad, nearly tripinnate fronds, its stiff, yellowish pin- 
1 Contributions from the U. §. National Herbarium, vol. 16, part 2, pp. 25-62, pls. 18-34, 1912. 
