TREE FEENS 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. Jiorrida 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. 1 



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. Jiorrida 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 Hemltelia differs from Eucyathea in having but "half an 

 indusium," so Alsophila departs still further in having no indusium 

 whatever, or hi 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 in Cyathea 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 pinnae of which are shown in plate 7; A. elongata, a species rang- 

 ing from Costa Eica to the Andes of South America, well marked 

 by its huge, broad, nearly tripinnate fronds, its stiff, yellowish pin- 



» Contributions from the U. S. National Herbarium, vol. 16, part 2, pp. 25-62, pis. 18-34, 1912. 



