PREFACE. 
During the past few years much study has been bestowed upon plants 
which furnish the rubber of commerce. This has shown that many of 
these are unknown botanically, that those which have been described 
have often been placed in wrong genera, and that the number of genera 
and species which may furnish rubber is apparently much larger than 
has been supposed. It has been discovered not only that several speties 
of the genus Sapium produce a part of the rubber of commerce, but 
that the genus is a very large one, and it will doubtless be found that 
more of its species are capable of yielding a satisfactory raw product. 
The accompanying paper on the species of Sapium of Mexico and 
Central America, by Mr. Henry Pittier, is therefore, in view of the 
growing demand for more rubber, timely, and the contents are such 
as to make it an important contribution to this subject. Most of the 
new species here proposed were first studied by Prof. Karl Schu- 
mann, but his death occurred before they had been published, or even 
manuscript upon them prepared. Mr. Pittier has described the new 
species recognized by Professor Schumann, together with two addi- 
tional species distinguished by himself, and has added, with appro- 
priate notes, descriptions of two already known. . 
J. N. Rose, Acting Curator. 
Ill 
