28 



THE VASCULAR SYSTEM OF THE AXIS 



[CH. 



having characters resembling those of the Osmundaceae are frequent in the Carboniferous 



Rocks. Whether the relation of these early Ferns to the Osmundaceae be a close one or 



not, the stelar structure oiDiplolabis corresponds 



to that to be shortly described in ThcDnnopteris^ 



a member of the Osmundaceous Series, while 



Metaclepsydropsis corresponds to what is seen 



in the more recent Osinunditcs Kolbei. Thus 



initial steps in meduUation in Ferns occurred 



in the Carboniferous Period. 



It is from the Osmundaceous Series of Ferns 

 that the most cogent evidence comes of the 

 origin of medullation. The recognition of the 

 Ferns in question, including the fossils, as a 

 phyletic unity is based primarily upon the struc- 

 ture of the leaf-trace, which is characteristic for 

 them but not for other types of Ferns. The cor- 

 rectness of this recognition has been doubted : 

 but those who object should produce evidence 

 of the existence of Ferns having the Osmun- 

 daceous leaf-trace which are not of Osmun- 

 daceous affinity. Till this is done the objection 

 cannot be held as valid. On this basis we are justified in recognising the70smun- 

 daceous type by means of its leaf-trace structure as far back as the Palaeozoic Period, 



ig. 12 1. Transverse section of shoot of 

 Tubicaiilis solenites Cotta, after Stenzel, 

 from the Permian, showing stem with pro- 

 tostele, and the last four leaf-traces travers- 

 ing the cortex. The leaves are numbered in ■ 

 their succession. The drawing is simplified 

 by omission of roots, etc. 



Fig. 122. Transverse seclion of stele of Mclackpsydropsis duplex, 

 to show inner tracheides and parenchyma, x^ router xylem; 

 x- = inner xylem; / = conjunctive parenchyma: slide 1109. 

 (X36.) Lower Carboniferous. (After Gordon.) 



