TREE FERNS OF NORTH AMERICA——-MAXON. 487 
THE TRIBE DICKSONIE®, 
Differences in habit and size of the plants and in the general shape 
and dissection of the leaves offer more obvious marks for the recog- 
nition of the three genera of Dicksoniex than do the indusia, iach 
are alike in position and of very similar structure. Thus, the several 
species of Culcita are small plants, none of them ever treelike, having 
short rootstocks only a few inches high and a spreading crown of 
broad, skeletonlike, greatly dissected leaves rarely more than 3 or 4 feet 
high, including the stipe; while Cibotium and Dicksonia are mostly 
if not altogether arborescent, plants at least of huge growth. In dis- 
tinguishing the latter genera it will be noted that the blades of Dick- 
sonia are elongate and either lanceolate or oblanceolate in outline, 
and that those of Cibotium are very much more ample and broadly 
ovate or even triangular. Coupled with these differences, however, 
are characters afforded by the sori, which, when once made out, are 
unmistakable. The position of the sori and the general structure of 
the double indusium has already been touched upon; the generic 
distinctions follow. 
THE GENUS DICKSONIA. 
In Dicksonia the outer concave lip of the indusium consists of the 
greenish leaf tissue of a small marginal lobule of the leaf segment 
which is slightly if at all modified by its function as a partial covering 
for the sporangia. The shape but not the texture of this outer lip is 
shown in plate 13, figure B, representing Dicksonia navarrensis, of 
Costa Rica and Panama. The inner lip, or true indusium, is seen to 
be of similar form, almost hemispherical, its hollow inner or under 
surface embracing the sporangia upon the side opposite to the outer 
lobe. It is yellowish and rigidly cartilaginous, thus conspicuously 
different in texture from the outer lip, and is attached along the base 
of the receptacle, upon which the sporangia are borne. 
But one other species of Dicksonia has been described from North 
America, the closely related D. lobulata of Costa Rica. This and the 
other members of the genus have like indusia. A section of a primary 
pinna of D. navarrensis is shown in plate 14 at natural size. The leaf 
tissue is rigidly coriaceous and of a yellowish green color. The trunks 
of this species, as I have seen them, are about 9 to 12 feet high and 
always roughly clothed with the very stout smooth bases of old 
fronds, a feature which seems to be characteristic of the genus as a 
whole. 
THE GENUS CULCITA. 
Although the indusia of Culcita are very similar to those of Dick- 
sonia, the plants are so different in habit, in their smaller size, and in 
the nondevelopment of an upright trunk, and bear quadripinnate 
