vii] STELE OF DEVONIAN PLANTS 91 



centric, the room for the development of primary tissues was 

 limited. In either case a stage was eventually reached when 

 further vascular development could only be provided for by 

 peripheral additions, i.e. secondary wood. This new type of 

 structure, as is well known, was often accompanied by a reduc- 

 tion in the primary scaffolding. 



In drawing attention to the existence in Devonian times of 

 a primitive type of stele, it must not be imagined that all 

 Devonian plants possessed such anatomy. On the contrary we 

 know of several arborescent types possessing secondary wood of 

 the modern type found among the higher plants, occurring in 

 the Middle Devonian of Scotland 1 and the Devonian of Russia 2 

 and the United States 3 and Canada 4 . It is thus clear that along- 

 side of Psilophyton there existed other plants which had already 

 reached a far higher stage of evolution as regards the stele than 

 we meet with in that genus. In fact Psilophyton was probably 

 one of the latest survivals of the primitive Devonian types 5 . 



1 Palaeopitys Millerii, M'Nab (1870). 



2 Callixylon Trifilievi ( =Dadoxylon Trifilievi), Zalessky (1909) and (1911), 

 p. 28, PI. IV, figs. 1-3. 



3 Callixylon Oweni, Elkins and Wieland (1914). 



4 Dawson (1871). 



5 [The Author left this chapter unfinished, and regarded it merely as a 

 preliminary draft. A. A.] 



