RAY TRACHEIDS IN THE CONIFERALES' 
RutTH HOLDEN 
(WITH PLATES I AND It) 
In classifying the various families of conifers from an anatomical 
standpoint, the structure of the medullary rays, whether fusiform 
or linear, thin or thick-walled, pitted or unpitted, has been a 
valuable criterion. Another feature which has not been emphasized 
as much as it deserves is the presence or absence of ray tracheids. 
DeBary, Mayr, PENHALLOw, and others have reported them in 
certain genera, but to reach any conclusion of phylogenetic 
importance it will be necessary to consider the conditions affecting 
their appearance in all the main groups of conifers, extinct as 
well as living. 
Extinct conifers 
Succiniferous Pityoxyla of the Lower Cretaceous as described 
by JEFFREY and Curyster (1) lack ray tracheids of any sort. In 
another less ancient cretaceous Pityoxylon, described by BAILEY (2), 
however, ray tracheids are present, both marginal and interspersed. 
Likewise in amber containing Pityoxyla from the Baltic deposits 
of the Late Eocene or Early Oligocene (3), marginal tracheids are 
present in the mature wood, though not in the early annual rings. 
In other types of fossil wood, namely Cupressinoxylon, Cedroxylon, 
Taxoxylon, Araucarioxylon, etc., ray tracheids have not been 
described. 
Living conifers 
ABIETINEAE 
JEFFREY (4) has divided the Abietineae into two groups: the 
‘Pineae (Pinus, Picea, Larix, and Pseudotsuga) and the Abieteae 
(Abies, Cedrus, Tsuga, and Pseudolarix). Ray tracheids are present 
normally in all four genera of the Pineae, attaining their greatest 
complication in the hard pines, where they are dentate and reticu- 
*Contributions from the Phanerogamic Laboratories of Harvard University. 
No. 48. : 
Botanical Gazette, vol. 55] ‘ [56 
