VII] 



MEDULLATION 



127 



leading up to the soleno-xylic state, a condition frequent among primitive 

 Ferns (Ophioglossaceae, Osmundaceae, Schizaeaceae). 



Such ontogenetic and comparative evidence of the intra-stelar origin of the pith 

 derived from Hving Ferns receives full support from comparison with early fossils. The 

 Botryopterideae and Zygopterideae, families of the Carboniferous Period, illustrate the 



Fig. 120. Transverse section of a stem oi Osmunda regalis, showing "mixed pith," and the 

 greater part of the xylem-ring. ( x about 75.) (G.-V. Coll. slide 1912). (After Gvvynne- 

 Vaughan.) The presence of tracheides scattered through the pith appears to have followed 

 on injury. It was held by Gwynne-Vaughan to support the theory that the pith of the 

 Osmundaceae is phylogenetically stelar, not cortical. Compare similar traumatic change 

 in the pith oi Botrychium ternatiini (Bower, A7in. of Bot. xxv, p. 544). 



initial steps from the solid xylem-core oi Botryopteris forensis, or of Tubicauh's (Fig. 121), 

 leading to the condition seen in Diplolabis Romeri, where the xylem is composed e.xclu- 

 sively of tracheides, but is differentiated into an inner zone of short reticulated tracheides, 

 and an outer zone of long pitted tracheides. In Metadepsydropsis duplex the inner zone 

 includes also parenchyma, which, with the inner tracheides, forms a "mixed pith" (Fig. 122). 

 This internal differentiation is entirely intra-stelar, and may be held as providing two 

 successive steps towards the establishment of a medulla. It is significant that sporangia 



