230 BOTANICAL GAZETTE [MARCH 
not in the region opposite. Thus it is seen that the place at which 
reversionary structures occur depends on the localization and 
nature of the hypertrophy. 
Root oF Q. RUBRA.—The ray structure of the roots of northern 
oaks is aggregate. Fig. 19 shows a transverse section of a root 
of Q. rubra in which the aggregate nature of the rays is apparent. 
The details of the aggregation may be seen to better advantage in 
fig. 20, which represents one of these rays under higher magnifi- 
cation. In the central part of the ray the organization is more 
parenchymatous than on the outer edges. The aggregate con- 
dition persists in the roots of the most mature trees, and is good 
evidence that the ancestral type of oak ray is aggregate, if it be 
admitted that the root more than any other organ longest retains 
primitive features. 
Fig. 21 is a tangential section through a root of Q. rubra, and 
the undoubtedly aggregate condition that it presents may be 
compared with the tangential aspect of ray organization in the 
stem as represented in fig. 4. 
SEEDLING.—The oak seedling in its younger stages has clearly 
developed aggregate rays. In the older saplings the aggregation 
may be noted in the first formed annual rings. In the successive 
annual rings the aggregate type passes over into the compound 
ray. No separate illustration of this is given because it resembles 
so nearly that described for the root. Seedlings of Q. rubra, . 
Q. velutina, and Q. alba all show aggregate rays, so it may be 
assumed that it is a general situation for the genus. 
REPRODUCTIVE AXIS.—The reproductive axis of the oak does 
not show aggregate rays. They have disappeared in the genus 
probably because there is no longer any definite localization of 
the acorn-bearing branches, as in the ovuliferous aments of birches. 
Species of oak of extra-tropical range have in general lost the catkin- 
bearing habit (in the case of the female flowers). 
SteM.—The situation in the oak regarding wounding is some- 
what different from that of the birch. The recovery from injury is 
much slower, and the conditions of atrophy are more marked in 
the region of the wound than those of hypertrophy. Aggregate 
rays (1), similar to those figured as normal in the root and seedling, 
