TREE FERNS OF NORTH AMERICA—MAXON. 481 
ean be devised, and to frankly recognize it as one merely of con- 
venience. To the objection that the recognition only of the three 
genera above mentioned is scarcely logical, since each of these is 
susceptible of division into two or more fairly distinct groups, it may 
be answered that at least the treatment here followed has the sanction 
of long usage and on this account will incur neither confusion of old 
names nor the substitution of new ones. The species constituting the- 
tribe Cyathez may be grouped, then, under the three genera Cyather, 
Alsophila, and Hemitelia. 
THE GENUS CYATHEA. 
The North American representatives of the genus Cyathea fall 
readily into two sections, based upon characters of the indusium; in 
the first of these the indusium is cup-shaped or saucerlike; in the 
second it is like a complete hollow sphere. Of the 50 species occurring 
in North America about 30 fall under the latter section. 
An excellent example of the cup-shaped or hemispherical type of 
indusium is seen in Cyathea elegans, of Jamaica, a species with large 
tripinnatifid fronds. Plate 9, figure A represents the young sori of 
this species at a stage when the sporangia are just reaching maturity 
and are beginning to crowd upward and outward, thus widening into 
a broad mouth the opening above them which is first evident merely as 
an apical pore. In plate 9, figure B is shown a later condition, in | 
which the sporangia have reached maturity and have mostly dis- 
appeared. Another example of similar but less pronounced form is 
shown in plate 10, figure A, representing the sori of Cyathea arborea. 
In this the indusium is saucer shaped, and the sporangia are seen to 
have been heaped high above its low even margins. In all species of 
the section of which these two species are illustrative, the indusia are 
firm, have perfectly entire margins, and almost invariably are per- 
sistent long after the spores have been shed and the sporangia drop- 
ped. The central position of the elevated receptacles may also be 
noted in the figures cited, as well as details of venation and pubescence. 
The sori are seen to be borne near the midrib (costule) of the segment, 
being seated at or just below the forking of the slender lateral veins. 
This section of the genus is known as Eucyathea or true Cyathea, 
since it contains those species which in general form and structure of 
the indusium agree with C. arborea, the type species of the genus. 
Of the 20 North American species all are confined to the West Indies, 
excepting only C. arborea, which occurs sparingly also on the continent 
from Mexico southward. 
A very different type of indusium characterizes the second section 
of Cyathea, which having once been regarded as generically distinct 
38734°—sm 191131 
