1910] THOM PSON—RAY TRACHEIDS 105 
practically every one of the high rays was formed by a combination 
of two or more lower ones. Only very rarely was a height of four 
cells attained otherwise. The so-called primary rays are then really 
formed secondarily, by fusion. To what extent this fusion is char- 
acteristic of the other forms has not yet been determined, but it would 
seem that the low ray is the primitive condition in the pines. 
TYVUR VO 00030 3I0 
} fi A, jo —- 
\ ee - —< i: z 
Uy 
alate aks Nm Tee 28. 
Fics. 4, 5.—Fig. 4, P. resinosa: young root; the transformation of short tracheids 
to interspersed ray tracheids, and the fusion of rays; fig. 5, P. Sirobus: young root; 
the replacement of ray tracheids by parenchyma; at x is a degenerating tracheid. 
The approach of the rays does not always end in the production of 
One with interspersed tracheids. When the rays have drawn together 
till the intervening tracheids have formed true ray tracheids or are 
on the point of doing so, then these elements are often replaced by a 
Tow of parenchyma cells which soon assume the ordinary form and 
size. In fig. 5 at the left are the short transitional tracheids which are 
replaced at the right by high parenchyma cells. Farther toward the 
cambium these gradually become lower and indistinguishable from 
other cells of the ray.* Instances were also observed where the 
* In this drawing, and in the others as well, with the exception of figs. 8, % 14-16, 
the simple pitting of the wall of the parenchyma has been used to distinguish this tissue 
from the elements with their bordered pits, the nuclear and protoplasmic 
_ Contents of the former being omitted. 
