62 BOTANICAL GAZETTE [JANUARY 
Conclusions 
o sum up, ray tracheids are present in Pityoxyla above the 
Middle Cretaceous; in all other fossil conifers they are absent. 
In living forms they occur normally in the Abietineae, traumatically 
in the Taxodineae and Cupressineae, but are invariably lacking 
in the Podocarpineae, Taxineae, and Araucarineae. Unless we are 
to assume that ray tracheids have developed independently in * 
these groups, it is evident that the Taxodineae and Cupressineae 
must be related to the Abietineae. PENHALLOw admitted the 
relationship, but considered the cupressoid conifers ancestral to the 
Abietineae. The principle that traumatic tissue reverts to an 
ancestral type appears to be too strongly established to need dis- 
cussion here. Applying this principle to the case in point, it must 
be seen that PENHALLOW has read the series in the wrong direction, 
and that the Taxodineae and Cupressineae are descended from 
rather than ancestral to the Abietineae. This conclusion might be 
questioned on the score that normally primitive regions, as first 
annual ring of seedling, root, cone axis, etc., fail to support the wound 
evidence by possessing ray tracheids. The answer to that appears 
to be simple: even Pinus, in which ray tracheids culminate, does 
not usually have them next the pith or in the cone axis. In other 
words, ray tracheids never made their way into the cone axis oF 
first annual ring of any of the Abietineae, Taxodineae, or Cupres- 
sineae, but were characteristic only of mature wood. Conse 
quently, now that they have become reduced in the last tw? 
families, they can be recalled only to the regions they once occupied 
normally. This conclusion as to the relative antiquity of the 
three groups, based on the distribution of ray tracheids alone, is 
corroborated by the conclusion reached by JEFFREY (4 and 
from a study of resin canals. ; 
The history of the development of ray tracheids, then, would 
be somewhat as follows. Up to the Middle Cretaceous, all Piy- 
oxyla had simple medullary rays composed entirely of parenchym@- 
In the Middle Cretaceous they acquired ray tracheids. Some time 
between then and the Tertiary, a family sprung from these Pityoxyla 
characterized by the possession of ray tracheids. This family is . 
the immediate ancestor of the Taxodineae and Cupressineae, while _ 
