CORDAITALES 



175 



Taking the group as a whole, the internal structure of the leaf is 

 highly xerophytic^^"* (fig. 185). The epidermal cells are rather thick 

 walled, and the hypodermal cells on both sides of the leaf have very 

 thick walls and are grouped into ribs. There is a strong bundle 

 sheath connected with the ribs of the hypodermal layer, and be- 



FiG. 185. — A, Cordaites anguloslrialus; B, C. rhombinervis; C, C. lingulahis; r, ribs of 

 thick-walled cells; m, mesophyll; 0, centrifugal metaxylem; i, centripetal metaxylem; 

 px, protoxylem; p, phloem; b, bundle sheath; c, elongated cells connecting bundles; pa, 

 palisade: A, X60; B and C, X50. — After Renault. i^s 



tween bundles the cells are often elongated, as in some transfusion 

 tissues. Some forms have a well-marked paHsade layer, as in C, of 

 fig. 185. The veins are mesarch, even when the stem is endarch. 

 Gordon's discovery of needle-like leaves in connection with 



