16 THE GENUS PHORADENDRON 
ordinary barriers to plant migration.* Like the similar European 
Viscum album, with its searce-definable races capable of effective germ- 
ination only on the host-species from which the seed came, our eastern 
P. flavescens though attacking a large variety of plants is usually found 
confined to a single host in a given region, and such experiments as have 
been made on it show that it can be transferred from one host to another 
with difficulty if at all. How far this may be concerned in the poly- 
morphism of this species and how far its like may serve to limit the 
dispersal of most species, is at present a matter of conjecture only. 
Viewed on broad geographic lines, the species of Phoradendron usu- 
ally oeeupy areas that present severally an assemblage of fairly uniform 
meteorologie features with limiting environment,—in this respeet agree- 
ing with most other plants and with animals. The regions in which the 
species of Phoradendron occur or which, like the great valleys of South 
America, separate them, are indicated on the accompanying map. Few 
species range throughout any one of these regions, and it is very rare 
for a species to reach from one into the other. 
TAXONOMIC SUMMARY 
Briefly summarized, the purely taxonomie part of my study of the 
genus leads to the conclusion that Phoradendron may be best divided 
into two primary groups, respectively constantly without and constantly 
with cataphyls on their foliage shoots: for the first I am using the name 
Boreales since its species alone are represented in the north; and for the 
other, Aequatoriales since only its species are found in the equatorial 
region. Species destitute of expanded foliage are found in each group 
in small numbers. Those of the first group are pubescent for the most 
part, while only two of the second group are more than papillately rough- 
ened. The Boreales appear to be strictly dioecious; the Aequatoriales 
for the most part, though not exclusively, are monoecious, usually with 
some or all of their spikes androgynous. 
So far as shown by the material now contained in the great herbaria 
at Washington, New York, St. Louis, Brussels, (where von Martius’ per- 
sonal herbarium is), Copenhagen, Kew, Munich (where von Martius’ 
official collection is), Geneva, Buda Pest, Prag and Dahlem, and in many 
smaller collections, I find a total of 277 differentiable forms of which I 
regard 240 as species, and of which 66, or 23 per cent., are of the Boreales 
and 211, or 77 per cent., are of the Aequatoriales. 
The distribution of the main groups (forms which oceur in more than 
*Hedgeock believes light to be a very important factor in determining their 
spreading,—Journ. Wash. Acad. vol. 3. p. 265; and Viscum is known to need light for 
germination. 
