PREFATORY 3 
INTRODUCTION 
PREFATORY 
My attention was seriously called to the need of a revision of our 
leafy mistletoes through inability to understand the basis of characteriza- 
tion that could admit to one species such different appearing plants as 
those from the southeastern, southwestern and arid United States—not 
to mention California and Yucatan—to which the name Phoradendron 
flavescens is currently applied. Among their manifold differences, a 
diligent search was made for characters; the types of related species and 
varieties that have been held to be differentiable from flavescens were 
examined ; and every form occurring in the United States was traced to 
the known limits of its range, sometimes south of our national border. 
In the course of this study it became apparent that the great conserv- 
atism of Engelmann, who seems never to have given this genus the care 
that marked his study of the related genus Arceuthobium or Razoumof- 
skya, had not only caused him to withdraw segregates of P. flavescens 
that he admitted at one time, but had reacted on his early colleague in 
the study of our southwestern plants, Torrey, to the extent of causing 
a number of mistletoes which had been designated in the Torrey herbar- 
ium as new species to lie there, as they still do, without publication. Ав 
political boundaries do not often form satisfactory limits to such a study 
as I had begun, I was quickly lured into an examination of the Mexican 
species which approach our border, and of others which reach into the 
field of these, so that no arbitrary geographic limit, even, could be fixed 
short of the Isthmus. : 
At the New York meeting of Ше National Academy of Sciences, in 
November, 1911, and at the meeting of the Academy of Science of St. 
Louis on December 18, 1911, the preliminary results of this study of 
the northern species were outlined, and this was followed at the Wash- 
ington meeting of 1912 by presentation to the National Academy of a 
revision of all of the forms of Phoradendron recognized as occurring in 
continental North America. As I was then on the eve of departing for 
a year in the great herbaria of Europe, this révision was withheld from 
immediate publication so that several obscure Mexican species could be 
cleared up certainly, through authentic specimens, and in the hope that 
they might be illustrated from the types. Though the admission of Tor- 
rey’s long neglected manuscript names had quite prepared me for an 
apparently inordinate increase in the number of differentiable species 
in the genus, I was not a little surprised to find, when casting my results 
