PTERIDOPHYTA 



213 



on the erect stems demonstrates that they were aerial and green. In 

 Asteroxylon the xylem was deeply lobed. The conducting tissues were 

 very simple. There was no secondary thickening. The leaves, where 

 present, were without veins, although the base of each leaf was con- 



FiG. 169. Psilophytales from the Devonian of Scotland. A, Asteroxylon mackiei; B, 

 Rhynia major. {After Kidston and Lang.) 



nected with the stele by a strand of vascular tissue constituting a leaf 

 trace. 



The sporangia were borne singly at the ends of the branches and not in 

 association with the leaves. They were relatively large (in some species 

 up to 12 mm. long, but usually smaller), cylindrical, and homosporous. 

 The sporangium wall was several layers of cells in thickness. As in other 

 pteridophytes, the spores were formed in tetrads. In Hornea the sporog- 



