BOTANY 



remarkably primitive type, apparently formed 

 directly from the stem-tip and without any 

 mechanism for dehiscence. A related genus, 

 Hornea, has since been described from the same 

 locality, and another type, Asteroxylon, whose 

 stems had essentially the vascular structure of a 

 simple modern Lycopod. The remarkably gene- 

 ralised character of these plants may be judged 

 from the fact that Rhynia has already been grouped 

 by different botanists with the Thallophytes (Algse), 

 Bryophytes (Mosses), and Pteridophytes (Ferns), 

 with all of which it seems to have relationships. 



The importance of these discoveries stimulated 

 Arber, shortly before his lamented death, to write 

 a book on the Devonian floras in which he correlated 

 all that was then known concerning Devonian plants 

 in its bearing on our understanding of the earliest 

 land-plants. He shows that Rhynia is probably 

 none other than the petrified condition of Psilo- 

 phyton, a plant described by Sir William Dawson 

 in the form of impressions from the Lower Devo- 

 nian of Canada as early as 1859. The scepticism 

 which long surrounded these and many other 

 rather obscure impressions described by Dawson, 

 Penhallow, and others from the Devonian of 

 Canada and Scotland is now largely dispelled ; Sir 

 William Dawson's early descriptions are seen to have 

 been for the most part extremely accurate, and the 

 great value of these early researches is now apparent. 



159 



