16 THE GENUS PHOEADENDEON 



ordinary barriers to plant migration.* Like the similar European 

 Viscum album, with its scarce-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 occupy areas that present severally an assemblage of fairly uniform 

 meteorologic features with limiting environment, in this respect agree- 

 ing with most other plants and with animals. The regions in which the 

 species of Plwradendron 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 taxonomic 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 differentiate 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 occur 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. 



