1921] WHITAKER—BIRCH AND OAK 233 
and also from the standpoint of plant pathology. They are also of 
interest because they suggest the possibility of producing orna- 
mental woods experimentally. 
2. Three types of rays, aggregate, compound, and diffuse, 
which persist contemporaneously in Casuarina, are characteristic 
of angiospermous trees. The aggregate seems to be the more 
primitive one, from which the diffuse and compound have been 
derived by different processes of evolution. 
3. Wound reactions in woody forms must be considered with 
reference to the conservative regions, the seedling structures, 
and fossil record, because only those structures occurring as a con- 
sequence of injury which have parallel conditions in these parts 
can be regarded as true reversions. 
4. Work on living and extinct Gymnosperms has established 
certain principles on the basis of which experimental investigations 
in angiospermous woods may proceed a priori. 
5. All reactions following wounding are not true reversions. 
In general extreme hypertrophy is not favorable to reversion. 
6. The details of wound reaction in the birch and oak are 
different. In the birch the wound cap is large, the hypertrophy 
being very marked. As a consequence the traumatic or rever- 
sionary features are not found in this region, ge in that part of 
the cylinder opposite the wound. 
7. Abies recalls marginal ray tracheids as a consequence of 
wounding. These are found in the regions remote from the wound 
and parallel the situation obtaining in the birch. On the other 
hand, the mode of appearance of traumatic resin canals is similar 
to that of the aggregate rays resulting from injury to northern 
oaks, as the reversionary resin canals occur in the immediate 
region of the wound cap 
8. In the oaks the woniiel cap is small and does not show hyper- 
trophy to any marked extent. Correlated with this, reversionary 
features appear in the wound cap proper, in contrast to the birches. 
In conclusion, I wish to thank Professor E. C. JEFFREY, under 
whose direction this investigation has been made, for advice, mate- 
rial, and the use of text fig. 1 from his recent book, The Anatomy of 
Woody Plants; also my father for assistance in securing material. 
Harvarp UNIVERSITY 
