Ig11] CHAMBERLAIN—CYCAD TRUNK 07 
many as three cone domes in a single transverse section of the trunk, 
and the arrangement of pith bundles resembles that described for 
Macrozamia. Since the question would be settled by a glance at 
a longitudinal section of a cone-bearing Macrozamia trunk, it is 
hardly worth while to speculate. 
HISTOLOGICAL STRUCTURE 
The living trunk of Dioon spinulosum cuts rather easily with 
an ax or machete, but is amazingly difficult to saw. Microtome 
sections of fresh material are not hard to cut, but transverse and 
longitudinal sections are likely to break at the large fragile medul- 
lary rays. A general view of the histology of the wood is shown in 
figs. 12-14. 
XYLEM.—The xylem, in the older parts of the stem, consists 
principally of very long tracheids, and is traversed by large and 
small medullary rays. 
Some writers state that there is no protoxylem in the adult 
cycad trunk, but the statement obviously rests upon a mistaken 
notion as to the character of protoxylem, such writers. assuming 
that only spiral and annular vessels should be entitled to the name, 
instead of applying the term to the first xylem differentiated in a 
bundle, without respect to the character of the markings on the 
cell walls. The adult stele of Dioon spinulosum is endarch, and the 
protoxylem consists of scalariform tracheids which pass gradually 
into the pitted tracheids with pointed ends, constituting the prin- 
cipal mass of the xylem. The transition is unmistakable, the 
scalariform markings, elongated pits, and typical bordered pits 
sometimes being found in a single tracheid. The bordered pits 
are multiseriate, two, three, and even five or six rows being found 
in a radial view of a tracheid, so that in radial sections the wood 
might be mistaken for that of Araucaria (fig. 15). Pits are occa- 
sionally found on the tangential walls, but they are not numerous 
and are irregularly scattered. 
Besides the tracheids with pointed ends, the xylem contains 
elongated cells with transverse walls (figs. 12 and 15-17). These 
at first are thin-walled and contain starch, but later may or may 
not become lignified and pitted. They are not uniform in length 
