1910] THOMPSON—RAY TRACHEIDS 107 
margin of the upper ray settles down to form regular ray tracheids 
while the other remains transitional. A repetition of the process in 
this row gives rise, beyond the figure, to a double series on the margin 
of the upper ray. In many cases, however, the second row is formed 
without the intervention of the transitional cells, this stage being 
either hurried over or completely omitted. 
The only kind of ray tracheid whose formation has not yet been 
described is that composing the wholly tracheidal ray. When the 
rays are far apart, two or more divisions may take place in the long 
tracheids instead of the usual single one. Then one or more rows of 
transitional cells are formed midway between the rays, with other 
rows above and below, that is, touching the rays. When the latter 
rows separate from the central ones, these continue as irregular 
tracheidal cells which gradually become regular, forming a com- 
pletely tracheidal ray. Fig. 7 represents only the ends of the series, 
a considerable space at the center being left out; in @ are two rows 
of irregular tracheids, the lower of which settles down to form the 
completely tracheidal ray seen in b. 
At the right in figure 7) another phenomenon is illustrated. This 
is the replacement of a tracheidal ray by a parenchymatous one. It 
is not an abnormality in this section, for 
numerous examples were seen. In fact Eee as 
it seems to be the common method of a 
origin for secondary rays, that is for ones 
which do not run to the pith. Owing to 
the yearly increase in the circumference 
of the wood, many new parenchyma rays 
must be formed if the number in a given 
area is to remain at all constant, and it 
seems easier for them to be produced 
from cambial cells which give rise to ray Fic. 8.—P. resinosa: young 
tracheids than from longer ones which oot; the development of a 
give rise to wood tracheids. ray tracheid at the cambium. 
In the development of some of the ray 
tracheids at the cambium further evidence is afforded of the origin 
of these structures from tracheary tissue. In fig. 8 the cambium is 
at the right. It has just given off a tracheid whose upper end is 
