The Adult Cycad Trunk

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 originas 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 medullary 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.