1915] CHRYSLER—RAYS OF CEDRUS 395 
2. Marginal row consists of tracheids, but these are replaced by 
parenchyma at the limit of annual rings: Cedrus. 
3. Many of the marginal tracheids have been replaced by paren- 
chyma, or have degenerated into ghosts, or have disappeared: Cedrus. 
4. Marginal cells typically absent, but occur sporadically, 
especially as a result of injury: Abies. 
Cedrus shows itself to be a particularly plastic genus, not only 
in the lack of uniformity of its ray structure, but in its response to 
wounding, as shown by JEFFREY (2). It would be interesting to 
ascertain whether wounded material shows reversionary stages in 
the rays, but my material affords no evidence on this point.. In 
marked contrast to Cedrus in this respect is the nearly related 
Pseudolarix. In no part of this plant which has come under my 
observation has any appearance of ray tracheids been observed, 
- and JEFFREY (loc. cit.) remarks upon the absence of wound reactions. 
The foregoing observations afford no support to the conten- 
tion of PenHALLOw that marginal tracheids have been derived 
from parenchyma. Nor can we agree with his statement (4, p. 107) 
“the rare occurrence of tracheids in Thuja, etc., is to be inter- 
preted as the first evidence of a tendency in development which is 
only fully realized at a later period.” Since these words were 
written evidence has been accumulating which shows that the 
series must be read in the opposite direction, and that the sporadic 
occurrence of ray tracheids in the Cupressineae represents the last 
Stage in disappearance of these cells. The chief evidence in this 
connection has been supplied by JEFFREY in his study of wound 
reactions, e.g., in Cunninghamia (3) and the observations on the 
tays of Cedrus point in the same direction. Further, there are 
Physiological grounds for opposing the view quoted above, for it 
is easy to see how a complete row of marginal tracheids can function 
in carrying water radially, but a few scattered tracheids on the 
margin of a ray, e.g., of Thuja, must be entirely useless, and hence 
are better regarded as vestigial structures which point back to the 
time when the ancestors of Thuja had a functional row of marginal 
tracheids. Again, the writer has previously shown (1) that in 
Juniperus, a genus which like Thuja shows occasional marginal 
tracheids, the latter are conterminous with erect cells of the phloem, 
and erect cells sometimes occur where no marginal tracheids are 
