1921] WHITAKER—BIRCH AND OAK 227 
Fig. 18 is a higher magnification of part of the region indicated 
by X, and is in decided contrast with fig. 17 in ray structure. Here 
the rays are aggregate in organization at the outer edge of the 
stem, in that part of the wood laid down after the stem had been 
wounded. The central part of the stem shows only diffuse rays 
similar to those in fig.17. These aggregate rays are true reversions, 
and the place at which they appear (opposite the wound) is 
significant for the birches. 
WOUNDED SEEDLING OF B. POPULIFOLIA. Singe aggregate 
rays are present as a typical feature of the normal stem wood of 
B. populifolia, it might be expected that they would die out in 
the region of the wound. This is precisely what does happen. 
Farther back, laterally, they make their appearance in the cylinder 
but not as traumatic features. 
SUMMARY.—Text fig. 2 fiieaanis diagrammatically the ray 
situation obtaining in the root of B. alba, and of the genus as well. 
In the Betulaceae the aggregate type of ray is found universally 
in the root, essentially a conservative organ. This ray is in 
definite relation to the root traces, and is the primitive or ancestral 
type of ray for the genus. 
Text fig. 3 represents a wounded stem of B. alba, and shows the 
comparative acceleration in growth of the wound cap and the rest 
of the cylinder. No aggregate rays appear in this region of marked » 
hypertrophy. The reversionary structures appear opposite or 
behind the wound, on either side of the stem, and extend in some 
instances to the extreme opposite side of the woody axis. The 
seedling condition is represented in the central part of the figure, 
the aggregate rays appearing in connection with the foliar segment. 
The leaf gap is shown in black. The aggregate rays related to the 
foliar segment persist only through two or three annual rings, and 
then gradually become diffuse, the diffusion taking place first on 
the sides of the rays. The reproductive axis is also figured in 
connection with the foliar segments by supposing that the aggre- 
gation does not give way to the diffuse type of ray as in the seedling, 
but continues aggregate out into the cortex. 
ConcLusions.—That the aggregate ray is the primitive one 
for the birches is borne out by the fact that it is the type of ray 
