104 BOTANICAL GAZETTE [AUGUST 
The description so far has dealt with the marginal ray tracheids. 
The interspersed ones appear about the same time; their origin is 
illustrated in fig. 4. At the left of the figure are the irregular short 
tracheids extending between two rays. Farther to the left, that is, 
nearer the pith, these rays are more distant and the transitional 
tracheids are longer. To the right the rays gradually approach and 
Fic. 3.—P. resinosa: young root; the transformation of short tracheids to 
marginal ray tracheids. 
the tracheids shorten to form the regular interspersed ray tracheids 
which continue to the cambium. Thus interspersed ray tracheids 
originate from the same kind of tracheary element as the marginal 
ones. In the latter case the rays separate and the tracheids form 
along their margin. In the former, the rays draw together and the 
tracheids shorten and take their place in the center of the resulting 
composite ray. 
The formation of this ray calls attention to an undescribed phenome- 
non apparently rather common in Pinus, namely the fusion of rays- 
As noted above, when the rays originate at the pith they are usually 
only one or two cells in height; but in the adult wood many of them 
are much higher. In the root wood up to twelve years old, careful 
observations were made to determine to what extent the increa 
height was due to fusion. This was done by following the rays in @ 
series of sections from the cambium to the pith. It was found that 
oh 
ie 
: 
e 
# 
fi: 
a 
ie: 
% 
wm 
ee 
Bel et een Re aero 
ig 
Wii ee a aS peat = nti sila op se Sea 
PE Ra Rh a Te Caen gS ee ee SE 
