XVIIl] GENERAL CONSIDERATIOBTS 267 



primary xylem. It is, however, noteworthy that Renault's 

 Stigmaria, in which centripetal xylem forms a prominent 

 feature, is attributed to Sigillaria Brardi, a species in which 

 the vascular cylinder of the aerial stem illustrates a later and 

 not an earlier phase in the replacement of centripetal by. 

 centrifugal wood. 



It would seem, as Lady Isabel Browne' says, that most 

 Stigmarian axes had reached a more advanced stage in 

 specialisation than is shown in the stelar structure of the aerial 

 shoots. The relatively greater and probably the more precocious 

 development of secondary xylem in Stigmaria than in Lepido- 

 dendron or Sigillaria may have some significance in relation 

 to the smaller amount of " old wood'' " (in a phylogenetic sense) 

 in their steles. 



As is pointed out in a later chapter, recent researches into 

 the anatomy of extinct members of the Osmundaceae by 

 Kidston and Gwynne-Vaughan have brought to light a striking 

 parallelism in evolutionary sequence between the Lepidodendreae 

 and the ancestors of Osmunda and Todea, the two surviving 

 genera of one of the most ancient families of ferns. 



There can be little doubt as to a very close relationship 

 between Sigillaria, Lepidodendron, and Bothrodendron. Sigil- 

 laria seems to have outlived Lepidodendron and Bothrodendron. 

 The two latter genera are recorded from Upper Devonian rocks 

 in several localities, Bothrodendron being particularly abundant 

 in the pre-Carboniferous floras of Bear Island and other parts of 

 the world. A remarkable stem described by Dr White' as 

 Archaeosigillaria primaeva from Upper Devonian shales of 

 New York is spoken of by him as "one of the most highly 

 developed representatives of a fairly distinct archaic group 

 foreshadowing the later genera Bothrodendron, Sigillaria, 

 Lepidodendron and Lepidophloios." The type-specimen, when 

 first discovered, consisted of an apparently unbranched stem 

 reaching a length of 5 metres. From the swollen basal part 



1 Browne (09) p. 25. 



2 Scott (02) uses the terms old and new wood in discussing the evolutionary- 

 sequence in plant steles. 



' White (07). 



