1910] THOMPSON—RAY TRACHEIDS 103 
A frequent peculiarity is the possession of arms (fig. 2) projecting 
radially either from the ends, or the middle, or from both, and meet- 
ing similar projections from their fellows. In this case the bodies 
of the cells are some distance apart. The walls of all these short 
tracheids are generally thinner, and their bordered pits much smaller 
than those of ordinary tracheids. 
The usual sequence of these cells and their transformations 
toward the cambium are illustrated in fig. 3, which is a camera lucida 
drawing of two consecutive sections from a series of the root of Pinus 
resinosa. The cambium in this figure is toward the right (this 
orientation has been preserved throughout my illustrations). On 
their first appearance, toward the left of the figure, these tracheids 
have the more regular form approximating ordinary tracheids. 
Farther out they become irregular, usually with the above mentioned 
projections; then divisions occur, forming two superposed rows. 
These soon separate, taking up their position along the rays (center 
of figure) and forming irregular marginal cells, which gradually 
become regular and assume the form of true ray tracheids. In the 
ray at the bottom of the figure, ray tracheids have already been 
formed, but in one at the top this process has not yet been completed. 
In Pinus Strobus the sequence is similar. Often in both, the series is 
not so regular as the one just described; some stages may be hurried 
over, some greatly prolonged. In the one drawn, the transformation 
has taken place much more quickly than is usually the case, for the 
transitional stages may often be traced through several years’ growth. 
That these elements are indeed transitional is further shown in 
fig. 3 (Pinus resinosa) by the development on their walls of the denta- 
tions characteristic of the ray tracheids of the hard pines. When 
they first appear the elements are quite smooth, but as they assume 
more the shape and character of ray tracheids they acquire these 
dentations. One may often find intermediate cells whose walls are 
partly smooth like tracheids and partly dentate like ray tracheids 
(center of fig. 3). 
It is evident that in the root of these forms we have a complete 
transition between short tracheids and ray tracheids; that by a 
process of division, of shortening, and of radial extension marginal 
ray tracheids have been produced from tracheary tissue. 
