¥- 
BENEDICT: GENERA OF THE FERN TRIBE VITTARIEAE 157 
stem and leaves, and as another distinguishing feature, possess a 
dictyostele, a more advanced type of stele than occurs in the most 
specialized of the true Vittarieae. As a further proof of their 
proper differentiation it may be mentioned that they are con- 
nected through a series of species with Polypodium of the group 
including P. serrulatum and P. marginellum, with which they agree 
in stem, scale, and spore characters, not to mention others. 
The clathrate scales are distinctive. (See PLATE 2, FIG. 9-II, 
17.) In these only the internal walls are thickened, the result 
being a latticelike appearance, whence the name ‘‘clathrate.”’ 
The superficial walls remain, but so thin and colorless as often 
scarcely to be apparent. In the majority of fern scales, the 
thickening is more even, and the scale usually appears concolorous 
and more or less opaque. 
The outline of the leaves, that is, simple and entire, is to be 
correlated with the venation, which, except in the three or four 
smallest species, is always simply reticulate. The reticulation is 
based in most cases either on the plan of a midvein with uniform 
lateral areolae (PLATE 2, FIG. 1, 3), or the midvein may be lacking, 
and then all the veins are uniform (PLATE 2, FIG. 6; PLATE 6, FIG. 
2). Consequently there are no predominating vein branches, and 
therefore no divisions of the leaf. It will be shown later that this 
areolate type of venation is derived from a free dichotomously 
divided type. The only exception to the rule of entire leaves, He- 
cistopteris, belongs near the bottom of the scale, and may be con- 
sidered to have remained stationary in the dichotomously free- 
veined stage which appears in the ontogeny of several species of 
which the early stages have been studied. The different venation 
patterns above Hecistopteris depend upon the presence or absence 
of a midvein and the number of rows of areolae. 
The soriation varies with the venation but is essentially of 
one type for all the species but one, Anetium citrifolium, and in 
this the aberrant type is found to have been derived from the 
usual plan. This statement may appear at first sight entirely 
unwarranted, but a survey of the whole tribe will show that the 
sporangia, with the one exception noted, are borne in lines usually 
of considerable but indeterminate extent along series of interlock- 
ing veinlets. In the simpler, narrowest-leaved genera, the lines 
