Bailey . — A Cretaceous Pity oxy Ion with Marginal Tracheides . 323 
name Pinus scituatensiformis , since it possesses highly resinous ray paren- 
chyma and abundant epithelium about its resin canals, characters which 
occur in Pityoxylon scituatense. 
Conclusions. 
The occurrence of ray tracheides in this Mesozoic Conifer throws 
interesting light upon the origin and phylogeny of these structures. As 
has been mentioned above, Jeffrey and Chrysler have pointed out that ray 
tracheides do not occur in Lower Cretaceous pines, nor in the primitive 
regions of modern pines, namely, the cone axis and the first-formed wood of 
the stem. From this they inferred that ray tracheides are of comparatively 
modern origin, and that in all probability they were evolved early in the 
Tertiary. The distribution of ray tracheides in our lignite confirms these 
writers in their conclusion that the absence of ray tracheides is a primitive 
condition, but shows that these structures were evolved during the latter part 
of the Cretaceous rather than at the beginning of the Tertiary. This is shown 
by the fact that ray tracheides are feebly developed even in the older wood 
of the stem, and do not occur during the first ten to fifteen annual rings. 
Among modern pines ray tracheides are often absent or feebly developed in 
the first few years’ growth of the stem, but the condition which we have 
described is a more primitive one and indicates the recent origin of ray 
tracheides in the plant under consideration in this article. 
There are two theories which have been advanced recently to explain 
the origin of ray tracheides. Penhallow 1 holds the view that these structures, 
at least in Pinus , have been formed from ray parenchyma cells by the 
thickening of their walls. The transition from thin- to thick-walled paren- 
chyma, which occurs in certain pines, culminates according to this writer in 
the formation of ray tracheides. VVe are not able to follow Professor 
Penhallow in this supposition, since, as has been shown by the writer, thick- 
walled ray parenchyma is the primitive condition in Pinus , and the grada- 
tions from thick-walled to thin-walled parenchyma occur in pines which 
are becoming specialized by the reduction in thickness of the parenchyma 
walls and by the formation of ‘ Eiporen which is a natural concomitant of 
the process. 
Thompson 2 has more recently advocated the theory that ray tracheides 
were originally derived from short tracheides which have assumed a hori- 
zontal position parallel to the axis of the ray. His investigations, however, 
have been confined to Pinus resinosa , Sol., and P. Strobus , two perhaps of 
the most highly specialized and most modern pines. Among the more 
1 Penhallow, D. P. : A manual of the North American Gymnosperms, ch. vi. Ginn & Co., 
Boston, 1907. 
2 Thompson, W. P. : The origin of ray tracheides in the Coniferae. Bot. Gaz., vol. 50, No. 2, 
Aug., 1910, pp. 101-16. 
