XXXVI] 



CYCADEOIDEA 



375 



With very few exceptions the stems of Cycadeoidea so far 

 described afford no satisfactory evidence of the presence of more 

 than one vascular cylinder in the main stem: in this respect 

 Cycadeoidea agrees with such recent Cycads as Dioon, Ceratozamia, 

 and Stangeria. In the stem of C. Jenneyana the secondary 



FIG. 508. Cycadeoidea Wielandi. Transverse sections of the secondary xylem 

 of the stem. A, the middle region; B, the inner edge of the xylem. (After 

 Wieland; x 100.) 



vascular tissue reaches a considerable thickness and shows signs 

 of 'rings' in the xylem, but these are probably the result of some 

 periodic interference with the uniform growth and not an indica- 

 tion of more than one xylem-cylinder. The secondary xylem 

 (figs. 508, 509) consists only of centrifugal tracheids with some 

 spiral elements on the inner edge ; though manoxylic. Cycadeoidea 



