424 
CARL S. HOAR 
These maybe catalogued under three heads — recapitulation, retention, 
and reversion. The principle of recapitulation is of much importance 
in the case of plants. Very often the seedling gives evidence of an 
ancestral condition which no longer prevails in the mature form. 
The principle of retention, similar to that of recapitulation, is particu- 
larly characteristic of plants. Scott first brought this to light in 
connection with the peduncle of the Cycads. More recently this 
principle has been greatly extended and it is now known that certain 
organs and regions of the plant often retain ancestral conditions lost 
in the more advanced parts. Thus the root throughout all plants is 
characterized by centripetally developed primary wood which now 
does not appear in the stem of any forms above the zoidogamous 
Gymnosperms. Other parts which often manifest retention are the 
reproductive axis, the first annual ring of the vegetative or reproduc- 
tive stem, the leaves, etc. The last principle, that of reversion, has 
been shown of late to be likewise of great importance. In this way 
conditions which are no longer present in our living forms may be 
recalled as a consequence of injury. In the consideration of the 
Betulaceae the validity of the general principles cited above will 
be assumed. 
Of the two families classified by Engler in the cohort Fagales, 
the Fagaceae are generally thought to be the higher. This family con- 
sists of several well known genera, among which may be mentioned 
Quercus, Fagus, Castan^ea, and Castanopsis ; and some of the anatomical 
peculiarities of these are here described. The genus Fagus is usually di- 
vided into two sections, namely, Eufagus and Nothofagus. The species 
of Eufagus (northern beeches) possess broad rays, while those of Notho- 
fagus (antarctic beeches) have rays of only one to two cells in width. 
In addition to the distinctions of ray structure the wood fibers of 
Eufagus have bordered pits in contrast to those of Nothofagus in 
which the pits are simple. In the latter also the fibers are sometimes 
septate. Castanea and Castanopsis show close agreement with regard 
to the wood. Narrow medullary rays occur extending outward from 
the angular pith. A feature of difference from the two genera last 
named is the presence in Quercus of hroad medullary rays. 
The second family of the cohort Fagales and the one dealt with 
especially in this work is the family Betulaceae. It includes the 
following genera: Alnus, Corylus, Carpinus, Ostrya and Betula. In 
the anatomy of their woods certain general features occur. Narrow 
