22 GYMNOSPERMS 



Hclerangiitm. — This stem is well known and is one of a more primi- 

 tive type than Lyginoptcris, for it has a protostele resembling that of 

 the living fern, Glcichenia (fig. 21). The vascular strands are inter- 

 mingled with thin-walled tissue. At the center, their tracheids have 

 bordered pits; but at the periphery, there are tracheids with spiral 



Fig. 18. — Lyginoptcris lontaxii: I, diagrammatic longitudinal section of a seed in its 

 cupule; II, III, IV, and V, show the structure in transverse section at the levels A, B, 

 C, and Z? of I; «, central part of nucellus, and n', sclerotic outer part of nucellus with 

 pollen chamber (/>) between; /, integument with outer stony layer shown in black; b, 

 vascular bundles; c, cupule. — .\ftcr Oliver and Scott. -i^' 



markings, the protoxylem, surrounded by other tracheids, so that 

 the strand is mesarch. The centrifugal tracheids have spiral or sclari- 

 form markings; the centripetal tracheids are pitted. The secondary 

 wood of the stele consists of tracheids with multiseriate bordered pits 

 (fig- 22). 



The cortex is differentiated into an inner layer of thin-walled cells, 



