STUDIES OF TROPICAL AMERICAN FERNS—NO. 6. 
By WILLIAM R. Maxon. 
INTRODUCTION. 
The present number of this series’ is devoted mainly to a consid- 
eration of three groups of Polypodium whose species have for the 
most part been greatly misunderstood. The positive identification 
of many of the species depends largely upon a set of characters which 
until recent years have received scant attention, namely, the points 
of distinction afforded by the scales of the rhizome, or of the lamina, 
or of both. The rhizome scales are, for example, of especial im- 
portance in the group of Polypodium trichomanoides, as shown here- 
after; and as studies of other groups within this genus have pro- 
gressed, it has become more and more clear that in the scales or 
pale of the rhizome are to be found in general the most definite, 
constant, and readily available characters of all for the distinction 
of closely allied species. In some groups, as in that of P. tricho- 
manoides, the very existence of these characters would pass unsus- 
pected in a casual examination because of the general similarity of 
the plants in gross morphology; and the confusion which long pre- 
vailed among the species of this particular group is in fact traceable 
largely to this circumstance. 
Besides the structural differences just mentioned, and correlated 
with them, there are peculiarities of habit, of venation and outline, 
and of several trichome structures of the frond which, under the 
former and more general method of study, passed as individual or 
regional variations but which evidently are to be regarded as im- 
portant substantiating characters. These may appear among related 
species in various combinations. In preparing keys primarily for 
the identification of species emphasis must, therefore, be placed now 
upon one set of characters, now upon another, preference naturally 
being given to those which may be made out readily; but any synop- 
tical treatment which pretends to completeness will certainly take 
account of scale structure in some detail. No further explanation 
need be offered for the frequent use and mention of relatively minute 
characters of this sort. 
* Contr. U. S. Nat. Herb. 10: 473-508. pls. 55, 56. March 30,1908. Ibid. 13: 1-438. 
pls. 1-9. June 30, 1909. Ibid. 16: 25-62. pls. 18-34. June 19, 1912, Ibid. 17: 133- 
177. pls. 1-10. June 20, 1918. Ibid. 17: 391-425. pls. 11-23. January 21, 1914. 
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