12 



MORPHOLOGY OF GYMNOSPERMS 



Students should be warned that it is not always easy to distinguish 

 metaxylem elements, especially in very young tissue and in the foliar 

 extremities of bundles. 



VASCUtAR ANATOMY OF CYCADOriLICALES 



The anatomical peculiarity of this group is that it displays the 

 vascular types described above as characterizing the ordinary ferns, 

 and in each case adds to them the formation of secondary wood. The 

 best known of these stems will be considered briefly. 



pet 



Fig. io. — Heterangium Grievii: transverse section of stem; x, central mass of 

 . primary wood; x', secondary wood beginning to form; next the phloem and pericycle; 

 ic, inner cortex {x in this zone is a sclerotic group); It, leaf traces; r, base of an adven- 

 titious root; oc, outer cortex, present only in places; pet, base of petiole; Xabout 5. — • 

 After Scott (87). 



Heterangium. — The stems referred to this genus occur from the 

 base of the Carboniferous into the Permian, one of the oldest and 

 best investigated species being H. Grievii (i8). The slender stems, 

 not over i . 5 cm. in diameter, are ribbed by the decurrent bases of 

 petioles. The vascular axis is a pfotostele (fig. 10), very similar to 

 that of Gleichenia pubescens (fig. 2), the single vascular cylinder being 



