ANATOMY AND PHYLOGENETIC POSITION OF BETULACEAE 429 
aggregate condition and is now characterized by the possession of the 
diffused type of ray in its mature organization. 
In the case of the Betulae there are extremely interesting facts. 
In Betula populifolia Marsh, aggregate rays are characteristically 
present more or less throughout the whole plant. Because it has the 
aggregate ray present everywhere, this species must be regarded as 
one of the most primitive of the birches. On the other hand, Betula 
lenta L., the black or cherry birch, appears to represent a higher degree 
of development; for in the normal stem aggregation is conspicuously 
absent. Figure 18 shows a transverse section of Betula lenta cut 
through the root near the root-trace. The two aggregate rays may be 
plainly seen clustered about the region of the outgoing traces. Again 
it will be noted that the root illustrates the retention of the ancestral 
characters of the plant. The European white birch, Betula alba L., 
and its variety the canoe birch, Betula alba var. papyrifera Marsh., 
likewise supply interesting data. Figure 19 is that of a transverse 
section of the stem of Betula alba cut from a small twig at the very top 
of the tree. In the center of the field a very noticeable and large 
aggregate ray is visible. Such rays are characteristic of the vigorous 
upper catkin-bearing branches of this species. Ancestral structures 
are well known to occur in reproductive regions; and the situation 
presented by the species under consideration affords a further illustra- 
tion of this important general principle. 
Figure 20 shows a transverse section of a seedling twig of Betula 
alba, var. papyrifera. It has been wounded at about the third annual 
ring and again near the end of the fifth year. As a result of the 
wounds the structure has reverted and has formed many large aggre- 
gate rays. This is another instance of the traumatic recall of ancestral 
types, a condition very common throughout the Conifers. Figure 21 
shows a small portion of figure 20 enlarged. It is clear that the 
majority of the rays are from two to three cells in width, i, e., of the 
diffused type. Upon either side, however, is a large aggregate ray 
which has come in subsequent to the wounds shown in figure 20. 
Figure 22 is that of a young twig of the same species cut in the 
vicinity of the leaf-trace. In the normal stem cut from the internode 
no large rays appeared. Nevertheless, around the lateral leaf-traces a 
tendency to aggregate occurs in the first annual ring and somewhat in 
the second, but beyond these only the diffused type is present. Figure 
23 is a transverse section of a twig cut from the extreme top of an old 
