PSILOPHYTOPSIDA 33 



known genus is Zosterophyllum itself, of which three species 

 have been described, one of them (Z. myretonianum) in 

 considerable detail."^ ^^ Its fossil remains occur in the Old 

 Red Sandstone of Scotland, and show that it grew in dense 

 tufts anchored to the ground by a tangle of branching 

 rhizomes. From these arose numerous erect dichotomous 

 branches, 15 cm or more in height and 2 mm in diameter, 

 cyhndrical and cuticularized, with a central vascular bundle 

 whose xylem tracheids bore annular thickenings. An Austra- 

 lian Upper Silurian species, Z. australianum, was similar,^® 

 but Z. Rhenanum, described from the Lower Devonian of 

 Germany,^* is said to have differed in having flattened stems. 

 For this reason, it is suggested that the German species must 

 have been partially submerged, as shown in the reconstruc- 

 tion (Fig. 4A). The way in which the sporangia were borne 

 is shown in Fig. 4B. The spikes varied in length from i cm 

 to 5 cm, with the sporangia arranged spirally upon them, 

 each sporangium being up to 4 mm broad. Dehiscence took 

 place by means of a transverse split in the sporangium wall. 



Psilophyton, the genus which lends its name to the groups 

 Psilophytopsida and Psilophytales, besides occurring in 

 Canada and the United States, has also been found in 

 Devonian rocks of Scandinavia, France and Belgium. 

 Psilophyton princeps^^ is the best known species. Fig. 5F is a 

 reconstruction of the plant. It grew to a height of about i m 

 in dense clumps, arising from a tangle of creeping rhizomes 

 covered with rhizoidal hairs. The aerial branches seldom 

 exceeded i cm in diameter and branched profusely in a 

 manner that was rather different from that of the Rhynia- 

 ceae, for many of the dichotomies were unequal. In this way, 

 some parts of the plant give the appearance of a sympodial 

 arrangement, with a main stem and lateral branches. 



The lower parts of the aerial shoots were clothed with 

 abundant outgrowths which have been variously described 

 as leaves, spines and thorns (Fig. 5G). Their tips appear to 

 have been glandular, they lacked stomata and vascular 



B 



