ANATOMY AND PHYLOGENETIC POSITION OF BETULACEAE 427 
when found in its natural habitat. Figure 5 shows a transverse 
section of the center of a twig in the region of the node. In the first 
annual ring a leaf-trace appears on the right with which is associated 
a vestigial aggregate ray. 
This species affords an excellent example of retention and of re- 
capitulation. The large rays, which have disappeared in the ordinary 
wood, appear again in the first annual ring at the node and around the 
leaf-traces. Figure 6, a transverse section of the wood of an old stem, 
shows that even in maturity it does not manifest aggregate rays. 
Wounding, as illustrated in figure 7, brings about a different situation. 
In the center several clustered rays occur, though in the regions 
lateral to these the ray structures are all uniseriate. In figure 8, a 
transverse section of a mature root, several aggregate rays appear in 
the outer rings and two such rays run completely through the field. 
In the study of this species several salient facts may be noted. 
In the first place, in the normal stem the region of the internode does 
not normally show aggregate rays. When it is cut in the region of the 
node, aggregate rays appear to some extent within the first annual 
ring and around the leaf-traces. An examination of the seedling root 
shows its rays to be all uniseriate. Yet, around the root-trace and 
somewhat after wounding, the aggregate ray reappears, and in the old 
root it is a permanent feature. 
Apparently Alnus mollis is a peculiar type, which, after starting 
to form aggregate rays, has reverted to the uniseriate condition. It is 
a well known fact that plants in a state of reduction often have ances- 
tral structures in the mature parts. Thus, although the root of the 
seedling exhibits no large rays, since they have reverted to the uni- 
seriate condition, in the root of the mature plant the aggregate ray 
reappears. The stem manifests a similar situation. The peduncle of 
the ovuliferous ament often presents aggregate rays in considerable 
numbers. The same condition is illustrated by the axes which support 
the peduncles, and, in fact, not infrequently in the early annual rings 
of any branches of vigorous development. 
Figure 9 is an illustration of the first few annual rings of a twig 
of Alnus Yasha, a Japanese species. Here again, although the mature 
stem shows no large rays, yet, as the section indicates, in the first 
annual ring and around the leaf-traces aggregate rays make their 
appearance. Obviously the species has suffered degeneracy in a degree 
somewhat similar to that of Alnus mollis. 
