154 CONTRIBUTIONS FROM THE NATIONAL HERBARIUM. 
by its function as a partial covering for the sporangia. In both 
these genera, however, the true indusium is the inner yellowish or 
yellowish brown lip, which is rigidly cartilaginous, that of Dicksonia 
being rounded and more nearly hemispherical than that of Cibotium. 
In the Dicksonieae the species are much more difficult to distin- 
guish than in the tribe Cyatheae, partly because there is a less. 
appreciable difference in cut of leaf, and partly from the almost 
complete absence of special structures (various types of scales and 
hairs) which serve to distinguish the species of the Cyatheae. More- 
over, the branching of the veins is more variable and more directly 
correlated with degrees of fertility. Upon superficial examination 
merely it would be possible to so arrange the American forms of 
Dicksonia in a single series that they might appear to represent but 
a single polymorphic “species,” with several outlying forms. Thus, 
to an unusual degree the study of this genus must be a comparative 
one, dealing with a large series of specimens from as many localities 
as possible. This is rendered difficult, however, by the fact that the 
genus is apparently represented in America by few species and that 
individuals are not only of infrequent occurrence but are found 
usually in inaccessible heavily forested regions. 
But two species of Dicksonia have been ascribed to North America 
in recent years, D. lobulata and D. navarrensis, both described from 
Costa Rica by Dr. H. Christ. The first appears to be a valid species, 
and the second to be synonymous with the South American D. 
gigantea Karst. To these are here added two more: D. karsteniana, 
long ago mentioned from Costa Rica by Karsten, and D. ghiesbreghtii, 
now described from Chiapas. Descriptions of all four will appear in 
_the second part of volume 16, North American Flora. There are 
indications of two other species also, as noted below. The several 
species now recognized may be separated by the following key: 
. KEY TO SPECIES. 
Veins usually 5 or 6 pairs; fertile veins simple or sometimes once 
forked. 
Fertile segments 4 to 4.5 mm. broad; fertile veins simple; 
sori about 1.5 mm. broad... ......2.........0000000- 1. D. lobulata. 
Fertile segments 3 to 3.5 mm. broad; fertile veins simple or 
the proximal ones frequently once forked; sori about 
1mm. broad....2 2.2... eee cece cece cece cece 2. D. gigantea. 
Veins 7 or 8 pairs; fertile veins mostly once or twice forked. 
Fertile segments 4 to 5 mm. broad; fertile veins once or com- 
monly twice forked ...............00 00.0 c cece cece eee 3. D. karsteniana. 
Fertile segments 3 to 3.5 mm. broad, appearing much nar- 
rower than the preceding, the lobes also narrower; fer- 
tile veins usually once forked...................-0--. 4. D. ghiesbreghtii. 
