PREFACE, VII 
The succeeding paper (part 6), by Mr. Hitchcock and Agnes Chase, 
Assistant Agrostologist, is supplemental to a revision of the genus 
Panicum published by the same authors in volume 15 of the Contri- 
butions under the title of “The North American Species of Panicum.” 
As stated in the preface to that paper, it was impossible at that time 
to offer a satisfactory treatment of the tropical species. Since then 
much additional material, including collections made by the authors, 
has been accumulated. The present paper includes the results of the 
study of these collections and presents a fairly detailed account of 
the genus in tropical North America. There are included 116 species 
and 3 subspecies. Of these, 9 species are described as new; 5 are 
South American species which were unknown within the limits of the 
area covered; 1 is a species recently described; and 4 are species that 
were cited as synonyms or listed as dubious species but are now con- 
sidered valid. In all 19 species are added, making the total for the 
genus Panicum in North America 216. Each species is accompanied 
by an outline map graphically representing the geographical distri- 
bution within the Tropics of North America. 
The seventh paper of the volume, the sixth of Mr. Maxon’s series 
on tropical American ferns, is devoted chiefly to a revision of three 
widely misunderstood groups of species of the genus Polypodium. 
There are included descriptions of several additional new species of 
Polypodium and notes upon several species of Notholaena. 
The final paper is by Mr. O. F. Cook, of the Bureau of Plant Indus- 
try, United States Department of Agriculture. In the course of his 
tropical American work Mr. Cook has made field studies of the charac- 
ters and habits of the well-known cacao tree and a related food tree, 
the patashte. Though both of these have usually been referred to 
the genus Theobroma, their differences have seemed to Mr. Cook so 
great as to require their separation, and he has elsewhere published 
the new genus Tribroma based upon the patashte. In the present 
paper he compares the two trees from an ecological as well as a taxo- 
nomic point of view and develops a practical suggestion for securing 
increased productiveness by a system of pruning. The characters 
of the two genera are summarized at the close in technical descrip- 
tions. 
Freperick V. CovILLe, 
Curator of the United States National Herbarium. 
