VIl] 



MEDULLATION 



129 



Unequivocal evidence of their existence in this Era is suppHed by the Russian Upper 

 Permian genera Zalesskia, and Thajunopteris (Seward, Fossil Plants, ii, p. 325). Com- 

 parison of their stem-structure with the ontogeny of the modern Osmundaceae offers 

 corroborative evidence of the intra-stelar origin of pith. Five steps in the elaboration of 

 the protostele have been recognised in these Ferns, and they appeared roughly in strati- 

 graphical sequence. They are: (i) a solid xylem-core ; (ii) a heterogeneous xylem, but 

 without pith ; (iii) a central pith surrounded by a xylem-ring; (iv) the same, but with the 

 ring interrupted ; and (v) an interrupted xylem-ring lined within by accessory phloem and 

 endodermis. Of these the first three are involved in medullation : the others will be con- 

 sidered later in this Chapter. 



jicr en ph px 



[23. Thatnnoptcris Schlechtcndalii, Eich. Part of a stele, 

 a = outer xylem ; i5 dinner xylem. Permian. (After Kidston 

 and Gvvynne-Vaughan. From Seward.) (X13.) 



(i) Kidston and Gwynne-Vaughan at the conclusion of their comparative Studies wrote 

 as follows {Fossil Osmtmdaceae, iv, p. 466): "We regard the Osmundaceae, as a whole, 

 as an ascending series of forms whose vascular system is to be derived from a primitive 

 protostele with homogeneous xylem." Though this is the condition of every sporeling, no 

 adult example of it was known in any Fern with an Osmundaceous leaf-trace till Dr Marie 

 Stopes described Osinundites Kidstoni {Ann. of Bot. xxxv, 1921, p. 55). Here the primary 

 xylem appears as a solid xylem-core, but flanked externally by secondary tracheides. It 

 seems probable that the latter, which are present in no other representatives of the family, 

 have solved the physiological difficulty of increasing size without enlarging the primary 

 xylem. Consequently the stele of this Fern retained the state of the sporeling. The fossil 

 is of relatively late occurrence, probably Cretaceous, and its secondary thickening may 

 be held as explaining the survival of so primitive a type to so late a time. 



(ii) Two species of Zalesskia from the Permian have been found to possess a solid 

 xyle'm in their relatively large steles, without any pith. But their structure is heterogeneous, 



