86 



GYMNOSPERMS 



The large ray, or leaf gap, is immensely larger, in Dioon spinulo- 

 sum 20 or more cells in width and 50 or more in height. The gap con- 

 tains the leaf trace and, above it, a mucilage duct (fig. 78). The re- 



FiG. 76. — Dioon spinitlosiim: longitudinal 

 tangential section of mature wood, showing 

 thin-walled cells of the xylem (/) containing 

 starch (s), the xylem tracheids, and the small 

 medullary rays containing starch and calcium 

 oxalate crystals; Xi2s. — After Chamber- 

 lain.'"* 



Fig. 77. — Dioon spimdosum: trans- 

 verse section of stem, showing cam- 

 bium (f) ; thin-walk-d cells (/), contain- 

 ing starch (.v) ; calcium oxalate crystals 

 (a); phloem with some thick-walled 

 tracheids (</); and medullary rays (r); 

 X 1 25. — After Chamberlain. •"» 



ticulate arrangement of the tracheids, characteristic of the cycads, 

 is well shown in the figure. 



The leaf trace, as it appears near the base of the leaf gap, is small, 

 but in the cortex the trace becomes very conspicuous. 1 he girdling 

 of leaf traces in the cycads has long attracted attention, especially 



