PREFACE. 
Volume 16 of the Contributions consists of 24 papers originally 
published in 14 parts. The first of four papers constituting part 1 
is a discussion, by Mr. William R. Maxon, Associate Curator of the 
National Herbarium, of tho systematic standing of Asplenium an- 
drewsii with a reprint of Professor Nelson’s description. The second, 
much longer one consists of a report, by Dr. J. N. Rose and Mr. Paul 
C. Standley, Assistant Curator of the National Herbarium, on a collec- 
tion of plants chiefly from the region between the Pinacate Moun- 
tains of Sonora, Mexico, and Tucson, Arizona. The plants were 
obtained by an expedition directed by Dr. D. T. MacDougal, of the 
Desert Laboratory of the Carnegie Institution. Determinations of 
the plant species collected are given, together with descriptions of 
a few which proved to be new. In the third paper Doctor Rose 
describes a new cucurbitaceous plant from the vicinity of Tucson, 
Arizona, and bases upon it a new genus, Tumamoca. The last 
paper, by Mr. R. S. Williams, of the New York Botanical Garden, 
consists of descriptions of several new mosses collected in Panama 
by Mr. Maxon, with notes on other interesting species. 
The second part consists of a third installment of the results of 
Mr. Maxon’s studies of tropical American ferns. The most impor- 
tant portion is a monographic treatment of the North American 
species of Hemitelia, subgenus Cnemidaria, a group which has 
received little attention from fern students for many years and the 
members of which have been determined generally under wrong and 
loosely applied names. With an ample amount of material at hand 
the problem became chiefly one of taxonomy. That it is possible to 
offer a solution of its difficulties is due in large measure to the courte- 
ous assistance extended by the curators of several Kuropean herbaria. 
Part 3 is devoted to a monograph of the North American species 
of Nymphaea, the work of Mr. Gerrit S. Miller, jr., Curator of the 
Division of Mammals, in the National Museum, and Mr. Standley. 
It contains descriptions and illustrations of all the species of yellow 
pond lilies known at present from the New World. Heretofore 
there have been recognized in this area about half a dozen forms; 
the number in the present paper is 19. Most of the new species come 
from the Southern States, mainly from the coastal plain of the Gulf 
of Mexico. The work was initiated by Mr. Miller about ten years ago 
and was carried far forward by him, but owing to interruption by 
other duties he has availed himself of the assistance of Mr. Standley 
Vv 
