22 THE PSILOPHYTON FLORA [ch. 



probably old shoots and the old shoots may have been sterile. 

 At any rate we think it probable that the degree to which the 

 emergences are visible depends partly on the age of the shoots, 

 though it may, as Dawson states, be also a very variable 

 character, depending perhaps on habitat. Our point, however, 

 is that a fossil is not justly excluded from the genus Psilophyton 

 merely on the ground that it has no visible emergences. 



We may also agree that Halle ^ was the first to discover the 

 vascular nature of the spiny type of stem {P. ornatum), but it was 

 shown by Dawson and has been more recently confirmed by 

 Kidston and Lang that the apparently spineless stem is also 

 vascular. The fructifications of what we here term P. princeps 

 (figured by Halle ^ under the name Dazvsonites arcuatus, sp. et 

 gen. nov.) are the best examples we know in the form of im- 

 pressions (Fig. 6). They are described as "terminal capsules of 

 a narrowly obovoid or short fusiform shape and usually 3-5 mm. 

 long." Spores have not been recognised in them, a fact which 

 has become immaterial in view of the fuller evidence of the same 

 organs which we now possess in the petrified state (Rltj/nia). 



We now reach the most recent contribution to the subject of 

 Psilophyton, and undoubtedly the most important yet made, 

 namely a recent account of a member of this genus published by 

 Kidston and Lang^ under the name Rliynia Gzoynne-V aughani. 

 For reasons which will be fully discussed a little later, we have no 

 hesitation in referring Rhynia to Psilopihyton and this species is in 

 all probability either P. prince2JS or P. elegans as here defined. 

 The great interest of these Scottish specimens, from a chert 

 bed, not younger than the Middle Old Red, at Rhynie in 

 Aberdeenshire, is that the plants are not only petrified but 

 complete. They occur in a most remarkable series of beds of 

 silicified peat, crowded with stems of this plant in situ. The 

 description of the habit and morphology of Rhynia given by 

 Kidston and Lang confirms in a remarkable manner the account 



^ Halle (1916). The xylem elements are described by Halle, as by Dawson, 

 as scalariform, whereas in Rhynia they are annular, but from impressions 

 and macerated material it must be very difficult to distinguish between 

 these two types of thickening. 



- Halle (1916), p. 25, PI. 3, figs. 1-9, PI. 4, figs. 18-21. 



* Kidston and Lang (1917). 



