1921] WHITAKER—BIRCH AND OAK 225 
been wounded has been more rapid than on the lower side where 
there has been no disturbing influence. 
The place in which reversionary structures make their appear- 
ance (in this case the aggregate rays) is usually not in the region of 
the wound nor yet in the wound cap, but in the region of slower 
growth opposite the wound. This fact will be more apparent when 
the microscopic features are discussed. 
ROOT AND VEGETATIVE STEM OF B. niGRA.—One of the most 
conservative organs of a plant is the root. Fig. 7 is a transverse 
section of a root of B. nigra showing two rays related to root traces. 
That these rays are aggregate in organization, and not diffuse, as 
are the leaf rays of the stem in figs. 1 and 2, may be seen by referring 
to a still higher magnification of these rays in fig. 8. Taking the 
root as an organ in which primitive features are long retained 
after they have been lost in the vegetative axis, it would seem that 
the aggregate ray is the primitive one for B. nigra. 
Another conservative region is the node of the stem, and 
ancestral features are often found here in connection with the 
leaf ray. Fig. 9 shows a part of such a stem in this critical region in 
transverse section of B. nigra. Although the normal adult stem 
is characterized by diffuse rays, the aggregate type of organization 
is present in the first formed annual ring, itself a conservative region. 
Root, SEEDLING, AND REPRODUCTIVE AXIS OF B, ALBA.—Passing 
from a species with typically diffuse rays, even in the reproductive 
axis, it is of interest to note the ray organization in significant 
regions of B. alba. As stated, the root which longest retains the 
older type of ray structure shows such a close resemblance to that 
of B. nigra in important anatomical features that for comparative 
purposes fig. 7 will illustrate the situation found in the root of 
B. alba sufficiently well. In both the ray organization is aggregate. 
Fig. 10 represents a seedling of B. alba in transverse aspect, and 
even under low power the ray structure may be noted as aggregate. 
A higher magnification to show the detailed organization of one of 
the rays is not necessary, as they are anatomically similar to that 
shown in fig. 12 (a leaf ray through the reproductive stem of the 
same species). Fig. 11 is a low power magnification of a repro- 
ductive axis of B. alba. Here the decided aggregation of rays in 
