VII] 



SECONDARY THICKENING 



137 



where it is of relatively feeble development (Fig. 1 19). It is more extensive 

 in the larger species, and in B. virginiamim the zone of secondary wood is 

 extensive (Fig. 129). That zone is traversed by parenchymatous rays com- 

 parable in their essentials with those of Gymnosperms and Dicotyledons. 

 Externally a zone of cork may also spring from the periphery of the cortex. 

 A similar condition found in a Palaeozoic fossil led Scott to its designation 

 under the name of Botrychioxylon. At the centre of its stele is a " mixed 

 pith," and outside this there is a zone of secondary wood. Periderm bounds 



Fig. 1 29. Transverse section of the stele of Botrychiuin virginiamim, showing 

 massive pith : radially disposed tracheides and medullary rays are evidence 

 of the secondary activity of the cambium which is seen surrounding the 

 xylem. (After Atkinson.) (x6. ) 



the cortex externally. Scott refers the plant to the Botryopterideae, with 

 the nearest relation to Metaclepsydropsis, in which secondary wood is also 

 occasionally found. Recently a further example of secondary increase has 

 been described by Dr Marie Stopes in Osmundites Kidstoni, probably from 

 the Cretaceous Period {A7m. of Hot. xxxv, 1921, p. 55). It shows a combina- 

 tion of a solid somewhat stellate protostele with secondary wood arranged 

 in seven bays between the rays of the star, and it has a typical Osmundaceous 

 leaf-trace. These features supply further structural links connecting the 

 characteristics of the Botryopterideae with those of the primitive Osmun- 

 daceae. 



