Ill] PSILOPHYTON 23 



of Psil<yphyton given by Dawson as far back as 1859, on which so 

 much scepticism has since been expressed. There is the same habit 

 (Figs. 1, 2, pp. 15, 16) — branched underground rhizomes attached 

 to the peaty soil by numerous rhizoids, and bearing erect aerial 

 shoots, eight inches or more in length and ranging from 6 mm. 

 to under 1 mm. in diameter. These shoots bore no leaves. They 

 occasionally branched dichotomously. They bore "small hemi- 

 spherical projections Avhich were more or less closely placed 

 without apparent regularity. On some of these bulges tufts of 

 rhizoid-like hairs were borne, while in other cases the projections 

 developed into adventitious branches,... some aerial axes ended 

 in large, elongate-pointed sporangia^." 



The most interesting matter in connection with these stems 

 is that the anatomy, which is very perfect^ preserved in silica, 

 is often but not always vascular. Here again we have an im- 

 portant confirmation of a fact which Dawson as early as 1859 

 had been able to ascertain from simple impressions of similar 

 stems. There is a small central strand of xylem (annular 

 tracheids), surrounded by phloem and externally by a wide 

 parenchymatous cortex and a well-marked epidermis. There is 

 no distinct endodermis or pericycle. Stomata^ occur on the 

 aerial stems, but they do not appear to have been frequent. Some 

 slender axes are also met with in which no vascular tissues are 

 developed. Sieve plates have not been recognised in the phloem 

 region. All the tracheids of the xylem are alike in size, there being- 

 no obvious protoxylem groups and no conjunctive parenchyma. 



The branches were vascular, but apparently their steles were 

 not connected with that of the main axis. 



The sporangia varied considerably in size, but seem to have 

 attained a length of at least 12 mm. Their walls were several 

 layers thick. They contained an enormous number of spores. 



As Dawson originally pointed out, these plants were land 



plants, and Kidston and Lang's^ account of the habitat of 



Rhynia as "a peaty soil practically composed of the decaying 



remains of the same species" agrees fundamentally with 



1 Kidston and Lang (1917), pp. 765-6. 



- Dawson (1871), p. 90, stated that stomata occurred but he did not 

 figure them clearly on PI. XI, fig. 129. 

 3 Kidston and Lang (1917), p. 771. 



