226 BOTANICAL GAZETTE [MARCH 
the foliar segment is a marked feature of the woody cylinder, in con- 
nection with contrast with that shown in fig. 9 for B. nigra. Fig. 12 
shows one of these aggregate rays under a higher magnification. 
Since the root, seedling, and reproductive axis of B. alba show 
the presence of aggregate rays, it may be assumed, on the basis of 
principles derived from the study of living and extinct Gymno- 
sperms, etc., that the aggregate condition is the primitive or 
ancestral one in these species. This interpretation will be used in 
the present connection for determining the traumatic responses in 
wounded stems. 
WOUNDED STEMS OF B. ALBA.—Fig. 13 represents a transverse 
section made through a wounded vegetative axis of B. alba. The 
marked acceleration in growth in the wound cap which appears 
at the top is to be noted, as well as the corresponding retarding on 
the opposite side. Even under low power the flutings in the annual 
rings formed after injury may be seen on the side of the stem away 
from or opposite the wound. ‘These crenulations represent aggre- 
gate rays, which are not present in the normal wood, and mark the 
position where reversionary or traumatic features appear in wounded 
birch stems. The excessive hypertrophy which is so marked a 
characteristic of wounded birch stems does not seem to be favor- 
_ able to reversion, since only diffuse rays are found in the wound 
cap itself. Fig. 14 is a tangential section through the region 
opposite the wound of the stem figured in fig. 13. ‘The undoubtedly 
aggregate nature of the rays is evident. Fig. 15 shows some of 
these rays under a higher magnification, and the aggregation of 
rays and fibers is even more apparent. 
WOUNDED SEEDLING OF B. PAPyRIFERA.—A species closely 
allied to B. alba, and having the normal wood structure of northern 
birches is B. papyrifera. Fig. 16 represents a transverse view of a 
wounded seedling of this species. The region where reversionary 
structures might be expected to occur is opposite the wound (X). 
Fig. 17 illustrates the condition in the immediate region of the 
hypertrophy. This is typical of the region of the wound cap in 
the birches in general. The ray organization in this instance is 
undoubtedly diffuse, and similar to that shown in the case of the 
normal stem wood of B. nigra (fig. 1). 
