1910] THOM PSON—RAY TRACHEIDS 113 
are structureless, shadowy outlines replacing transitional tracheids, 
which they resemble in form, although often somewhat more irregular. 
They occur in considerable numbers wherever the former elements 
are found, and are evidently to be regarded as “degeneration prod- 
ucts,” to which transitional structures are always subject. As such 
they emphasize the transitional character of the regions in question, 
and help to complete the chain of evidence for the origin of the ray 
tracheids from tracheary tissue. 
PENHALLOW’s argument (1) for his theory that ray tracheids are 
derived by modification from the parenchymatous cells of the ray, is 
the occurrence of ray 
eee 
occurred. J must con- | 
firm PENHALLOW’s ob- ihe 
servation, however, Fic. 16.—P. serotina: medullary ray showing (1) 
especially in the young the ray tracheids with dentate walls, (2) the structure 
plant, where such an of the parenchyma cells, (3) tracheids conterminous 
: with parenchyma cells.—From PENHALLOW (1). 
tracheids conterminous S© ||P 1 
bi gam cells. | 8 OTEIR HI Al aa 
ut this is so rare that 0% ||} 0S © 
DrBary (8) was led to PIS PIS IC, 
assert that it never | Qe BIRSHe ge 
fae Ru oT | : 
ROSY 
appearance is more 
common than in the adult. I have observed and figured this feature 
in the formation of secondary parenchymatous rays from wholly 
tracheidal ones (fig. 7), and in the replacement of interspersed ray 
tracheids by parenchyma (fig. 5). I have observed also a similar 
Teplacement of marginal ray tracheids by parenchyma cells. In these 
instances, however, it is to be noted that the parenchymatous cells 
are toward the cambium (to the right in the figures), and so 7 eplace 
‘racheary elements, the reverse of what is required from PENHALLOW’S 
point of view. This is indicated in the very figure which PENHALLOW 
uses to illustrate the fact that ray tracheids may abut on parenchyma 
cells (his figure is reproduced herewith, fig. 16). The direction in 
which the oblique end walls are inclined, and in which the tail-like 
Projection points, which is seen on one of the ray tracheids on the top 
of the ray, indicates, as shown above (figs. 8-10, 12), that the cambium 
is to the left of his figure. The parenchyma cell which is in line 
