1921] WHITAKER—BIRCH AND OAK 221 
A comparative anatomical study of existing and fossil plants, 
especially the Gymnosperms, has shown that certain parts of 
plants (as the root, leaf, first annual ring, and reproductive axis) 
may have a different organization of tissues from the stem, which is 
more highly specialized. This fact has been responsible for the 
theory of retention, or, as it has been more recently called, the 
doctrine of conservative organs (7 
The third principle is based on the fact that upon injury certain 
structures and types of organization appear which are characteristic 
of older forms and more conservative regions of the plant. It is 
less well understood than the other two, and is capable of more 
misinterpretation. It is the one of chief interest in the present 
investigation. It should be pointed out, however, that only those 
structures occurring as a result of wounding which are comparable 
to conditions presented by the seedling and the conservative 
organs (root, leaf, etc.) can be relied upon in the interpretation of 
wound reactions. 
Experimental work on woody tissues is of interest not only 
from a general biological standpoint, but is also of importance from 
the point of view of plant pathology. It may be pointed out also 
that such investigations are of interest from an economic point of 
view, since they suggest the possibility of producing experimentally 
valuable ornamental woods. 
As the subject of wound reactions is a large and complicated one, 
for the purposes of brevity and clearness I have confined myself 
to those traumatic features which are connected with ray structures 
only. Other reversions and reactions consequent upon injury in 
the birch and oak, therefore, may be conveniently postponed 
until a later date. 
Ray organization in Angiosperms _ 
The three types of broad rays characteristic of the angiosper- 
mous forest trees are all found in different species of the isolated 
and probably ancient genus Casuarina. A synoptic diagram illus- 
trating these types as seen in Casuarina is given in text fig. 1. The 
simple or uniseriate ray which is a feature of the wood organization 
of the Conifers is also found in Angiosperms, and its occurrence 
