488 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1911. 
leaf blades of such distinctive form and cutting, that their recogni- 
tion as a genus seems justified. There are about half a dozen species, 
mostly of the South Sea Islands and Malaya, one, however, occurring 
in the Azores, Madeira, and Teneriffe, and another (Culcita conwfolia) 
inhabiting the high mountain forests of Jamaica and Santo Domingo, 
and extending on the continent from Mexico to Ecuador and Brazil. 
A pinnule (tertiary division) of the last is shown in plate 12, figure C. 
THE GENUS CIBOTIUM. 
The most highly differentiated indusium structure of any of the 
Dicksoniee is exhibited in Cibotium; for in this genus the outer lip, 
like the inner, is manifestly cartilaginous, and the two valves fit 
together like a box and its lid, offering complete protection to the 
sporangia until maturity. Plate 12, figure D, represents at four times 
natural size a section of a pinnule of Cibotiwm Schieder, of Mexico and 
Guatemala, in which may be noticed both closed and open indusia; 
as also in plate 15, which shows part of a primary pinna of the same 
species, at natural size. Three other species, all with deeply tripin- 
natifid or tripinnate leaves of similar form are known from Mexico 
and Guatemala. In these, as in most foreign members of the genus, 
the double indusia after maturity are bent back in a close row upon 
the leaf tissue of the underside of the segments, their position (1. e., 
whether oblique or parallel to the midvein of the segment) at this 
stage varying according to the species. Their shape also is diag- 
nostic, some of them having the valves sprung widely apart, like gaping 
jaws. In all but two or three of the species, both American and for- 
eign, the under surfaces of the blades are whitish or even bluish 
pruinose, from a dense covering of very minute and closely set 
whitish papille which are of a waxy nature. Not a few species of 
the tribe Cyathex also have a similar development, whose function 
must: be to guard against excessive transpiration. 
Cibotium has never been collected in South America, and is cer- 
tainly not common in the North American Tropics, judging from the 
small amount of material collected. We have also only scant and 
and conflicting data as to habit, from which it appears doubtful if 
the four American species normally attain a pronounced arborescent 
form, similar to that of the Hawaiian species, for example. A brief 
critical account of these, with notes on their rather involved taxo- 
nomic history and with illustrations of each, has recently appeared.’ 
DIFFICULTIES OF STUDY. 
From the foregoing it will be evident that tree ferns are more than 
ordinarily difficult of study on account of their enormous size. Even 
for the most commonplace data as to dimensions of trunk and shape 
1 Contributions from the U. S. National Herbarium, vol. 16, part 2, pp. 25-62, pls. 18-34. 1912. 
