1921] WHITAKER—BIRCH AND OAK 229 
normal wood, loses them as a result of wounding except in connec- 
_ tion with the appendage, when they may appear in the wound cap. 
Abies 
It will be helpful to summarize the wound reaction situa- 
tion in Conifers. From the point of view of wound reactions two 
features are significant, resin canals and ray tracheids. The 
Abieteae show a simplification or reduction of wood organization 
in lacking both of these characters in the normal wood (3). The 
root, however, has resin canals as a normal feature of its structure. 
In Abies, which has neither resin canals nor marginal ray 
tracheids in the normal stem wood, the situation in regard to 
reversion is interesting, because both resin canals and ray tracheids 
are recalled as a result of injury. There is a significant feature, 
however, in connection with the reappearance of these traumatic 
characters which parallels the situation obtaining in the angio- 
spermous groups under consideration. It has been pointed out by 
THompson (8) that marginal ray tracheids which are not normally 
present in the fir may be recalled as a consequence of injury, 
and that reversion takes place opposite the wound. JEFFREY (6) 
had earlier shown that in the case of a wounded stem of Cunning- 
hamia sinensis marginal ray tracheids, which are not a feature of 
normal stem organization, are recalled, and that these make their 
appearance opposite the wounded region. 
The appearance of traumatic resin canals as a result of injury 
is of special interest from the fact that they do not reappear in 
the region opposite the wound, as do the marginal tracheids, but 
in the wound cap itself. This condition, as will be evident later, 
parallels the situation in Quercus in connection with reversionary 
ray structures. 
Oak 
WovunpED stEms.—Fig. 6 represents the polished end of a 
wounded oak log Q. rubra. In this instance the healing has not 
been so complete as it was in the birch, and therefore the wound 
cap is restricted to the sides of the actual injury. This would be 
expected as a result of the slow growth of the oak, and accordingly 
reversionary structures will be expected in the wound cap itself, and 
