AS i iy a 
THE ORIGIN OF RAY TRACHEIDS IN THE 
CONIFERAE 
W. P. THOMPSON 
(WITH SIXTEEN FIGURES) 
Considerable attention has been devoted in recent years to the 
phylogenetic significance of ray tracheids in the Coniferae. PEN- 
HALLOW (1), arguing from their distribution and from his theory 
of their origin, concludes that the number in a given species is in 
direct proportion to its specialization, and that the forms where they 
are most numerous are derived from those where they are not so well 
developed. JEFFREY (2), on the other hand, from a study of certain 
traumatic phenomena, considers that the forms where they attain 
their greatest development, namely the pines, are the most ancestral, 
and have given rise by degeneration to the ones in which they occur 
sporadically. Apart from its intrinsic interest, it is hoped that the 
present study, by determining the origin of the ray. tracheid, will 
supply a basis for its correct phylogenetic interpretation. 
In carrying on the study, a thorough investigation was made of 
the character and mode of formation of the ray tracheid throughout 
the individual plant, but more especially in the primitive regions: 
seedling stem and root, young branch and young root of the adult, 
and the axis of the seed cone. The forms chosen for detailed investi- 
gation were our indigenous species of the hard and soft pines, Pinus 
resinosa and P. Strobus, but the results were confirmed in many other 
forms. The usual method was to follow a ray by means of a series 
of sections, from its beginning at the pith through the wood to the 
cambium. 
The origin of the medullary ray at the pith has been described and 
figured by Kny (3) for Pinus silvestris. He states that in this region 
all the ray cells are parenchymatous, and elongated not radially but 
vertically. These long cells are often in connection with similar cells 
from rays lying above or below. Very soon they separate, shorten in 
the vertical direction, and elongate radially to form the typical medul- 
lary rays, which during their further course are separate. A later 
tor [Botanical Gazette, vol. 50 
