430 
CARL S. HOAR 
and vigorous tree and illustrating a condition similar to that shown 
in figure 19. Aggregate rays are not noticeable in the mature vegeta- 
tive stem of this species except in relation to the leaf-trace and in the 
first annual ring. They are conspicuously present for a number of 
years in the seedling stem of Betula alba (sensu strictiore), but are not 
found even in the first annual ring of the seedling of B. alba, var. 
papyrifera. The wounded seedling of the variety does, however, show 
aggregate rays strongly developed as a result of injury. The vigorous 
catkin-bearing branches of this species Hkewise often manifest the 
aggregate ray clearly present, and figure 24 illustrates the interesting 
conditions which are found in the root. On the upper side of the 
figure appears a denser region of the wood increasing in breadth from 
the primary wood outwards. This is an aggregation of rays related 
to the trace of an outgoing secondary root. 
Conclusions 
On the whole the genus Alnus most clearly illustrates the condi- 
tions which are apparently primitive for the Betulaceae. In Alnus 
japonica, for example, we find congeries of somewhat enlarged rays 
related to the appendages (roots, leaves and lateral branches). These 
clusters of rays present in several features a contrast to the general 
ray conditions in the wood. First of all, the members of such ray 
groups are individually lengthened or broadened or both in contrast 
to the uniseriate organization of the rays of the mass of the wood. 
Secondly, they show more or less pronounced tendency to fuse with 
one another. Thirdly, the vessels which characterize the longitudinal 
structure of the normal wood are conspicuous by their absence in the 
grouping of rays under discussion. This condition of organization is 
well illustrated by Alnus japonica as figured in plate I. In other 
species of Alnus the tendency to grouping or aggregation of rays with 
the concomitant peculiarities described above becomes obsolescent, 
being retained only in regions of greater conservatism and susceptible 
to recall as a result of injury. Alnus mollis, Alnus rugosa, Alnus 
incana, Alnus maritima, etc., etc., show more or less marked degeneracy 
in these particulars. 
In the genus Corylus the condition is simple. As figures 10 and 
1 1 indicate, there is a distinct tendency towards aggregation. 
In the genus Carpinus we apparently have a very interesting 
situation. Here in most species the wood structure is characterized 
