rgr1t] CHAMBERLAIN—CYCAD TRUNK 103 
other. The distinction would doubtless be mu more difficult 
in young trunks than in old ones. 
The structure of the trunk of Dioon spinulosum is remarkably 
like that of the Bennettitales. The growth rings resemble those 
of Cycadeoidea Jenneyana, as described by WIELAND; the phloem, 
with its numerous thick-walled fibers in transverse section, is very 
similar to that of Cycadeoidea Wielandi; and the xylem, as it 
abuts upon the pith, also resembles that of Cycadeoidea. But 
there are also contrasting features; the broad medullary rays are 
not figured in the Bennettitales, and in the xylem cells with trans- 
verse walls seem to be lacking. 
Summary 
1. The paper deals with field material of adult stems of Dioon 
spinulosum, D. edule, Ceratozamia mexicana, and Zamia floridana, 
particular attention being given to Dioon spinulosum. 
2. In Dioon spinulosum the xylem zone in a plant 6 meters in 
height reaches a width of 10 cm., far exceeding the extent of any 
xylem zone previously described for any cycad. 
_ 3. Dioon spinulosum and D. edule have growth rings, which in 
D. spinulosum correspond to the periods of activity which result 
in the formation of crowns or cones, but which in D. edule do not 
correspond to such periods. No growth rings were found in 
Ceratozamia mexicana or Zamia floridana. 
4. Cone domes in the pith were studied in the four species. 
5. The histological character of the adult stem was studied in 
Dioon spinulosum. The protoxylem consists of scalariform 
tracheids, from which there is a gradual transition to the tracheids 
with multiseriate bordered pits, constituting the principal part of 
the wood. There are also cells with the same origin as the pitted 
tracheids, but with transverse walls which may remain thin-walled 
and contain starch or may become lignified. Besides the leaf 
trace bundles, scalariform tracheids are found in the large medul- 
lary rays. 
6. Both in the general appearance of the transverse section 
and in histological characters the adult trunk of Dioon spinulosum 
resembles that of Cycadeoidea. 
