THE ANATOMY AND PHYLOGENETIC POSITION OF THE 
BETULACEAE* 
Carl S. Hoar 
According to the system of Engler the Betulaceae are placed 
together with the Fagaceae in the cohort Fagales, which in turn is 
assigned to a position near the bottom of the Archichlamydeae. At 
the beginning of the Archichlamydeae is found the cohort Verticillatae, 
containing but one family, the Casuarinaceae. The Casuarinaceae 
are represented by a single genus, Casuarina, and the species of the 
latter occur in the Australasian and East Indian regions. 
In this work it is my purpose to treat anatomically and phylo- 
genetically the family Betulaceae. First, however, in order better to 
understand the situation with regard to this family, it seems best to 
treat somewhat in detail the family Casuarinaceae which is in close 
anatomical and phylogenetic relationship with the above. 
The position of these lower famiHes, as has been stated, is the 
one given by Engler and generally followed. However, since the 
origin of the Angiosperms is still somewhat in doubt, opinions with 
regard to the true evolutionary position of the various lower members 
of the Dicotyledons naturally vary. Evidence for one view is favored 
by one organ while perhaps another may give just as convincing 
evidence for an opposite conclusion. It is only when everything is 
taken into consideration that one can arrive at a proper understanding 
of the true situation. 
Among the various opinions there are two in particular which 
stand out from the others. The first is that of an origin from the 
Cycadales, while the second is one from the Coniferales through the 
Gnetales. Both of these views were based until recently almost 
exclusively upon the reproductive structures. Of late, however, 
Thompson (1912) has undertaken a study of the Gnetales with the 
vegetative anatomy as his object and has already published his results 
as found in the genus Ephedra. His research brings forward evidence 
* Contribution from the Laboratories of Plant Morphology of Harvard 
University. 
415 
