ANATOMY AND PHYLOGENETIC POSITION OF BETULACEAE 43 1 
by rays not uniseriate as in Alnus, but two or three cells wide and 
separated from one another by the usual longitudinal elements, namely, 
vessels, fibers and wood parenchyma cells. In contrast to this organi- 
zation of the wood which may be justly considered to be normal, we 
find clustered rays not opposed as in Alnus to those of the general 
wood structure but distinct because of their clustered condition as 
well as by reason of the absence among them of the longitudinal 
elements belonging to the category of vessels. It has been made 
clear at the beginning of this article in connection with the genus 
Casuarina that the type of dicotyledonous wood characterized by the 
uniform distribution of rays not uniseriate but usually of mediocre 
breadth (two to many cells) is the result of the diffusion of rays orig- 
inally grouped, throughout the woody cylinder. The special interest 
of Carpinus is that it presents this condition and that of aggregation 
from which it has been derived, side by side. In Carpinus cordata 
aggregate rays are found only in the earlier annual rings, disappearing 
in the adult. On the other hand, in Carpinus caroliniana and Carpinus 
hetulus the aggregate and diffuse ray conditions persist together in the 
mature structure. 
The wood organization of Ostrya is very similar to that of Carpinus 
cordata. In the mature wood of the stem grouped rays have entirely 
disappeared, but vestiges of them may be discerned in the earlier 
annual rings adjacent to the pith. The aggregate rays also persist in 
this genus in the root. 
The genus Betula is of peculiar interest on account of the large 
number of species and the considerable variety of wood structure which 
they present. In the common gray birch, Betula populifolia, of the 
Eastern States, we find diffuse rays and aggregations of rays close 
together. In Betula alba the congeries or aggregations of rays are 
found in the first annual ring of the vegetative stem, during a number 
of years in the seedling stem, and even more strikingly in vigorous 
reproductive branches. Aggregations of rays are likewise a feature of 
normal structure in the root and are related to the secondary roots. 
In Betula papyrifera (Betula alba, var. papyrifera) the occurrence of 
aggregations of rays is much more restricted. They are absent in the 
seedling, but are normally found in the root. They may be recalled 
in the seedling stem though not in the adult as a consequence of injury. 
In Bettda lenta and Betula lutea the condition of aggregation is clearly 
present only in the root. It is thus apparent from this summary 
