106 MEDULLOSEAE [CH, 



Myeloxylon (Medullosan leaf -stalks). 



1832. M edullosa elegans Cotta. 1865. Stenzelia Goeppert. 1876. Myelo- 

 pteris Renault 1 ; 1877, Aulacopteris Grand'Eury 2 . 



There is a very close agreement in general anatomical structure 

 between the numerous specimens of Myel'oxylon from the Permian 

 strata of Saxony and France and the Coal Measures and Millstone 

 Grit of England 3 ; the genus is also recorded from the Upper 

 Carboniferous of Kansas 4 . Two well-defined types instituted by 

 Renault are, however, readily distinguished by the form of the 

 hypodermal stereome strands. Myeloxylon may be defined a& 

 follows: Oval or cylindrical branched axes, reaching a diameter 

 of 15 cm., bearing pinnae having the characters of Alethopteris,. 

 Neuropteris, Odontopteris, and some other genera that were formerly 

 classed as Ferns. Below a single-layered epidermis, in which 

 stomata have been recognised, occur a few layers of parenchyma : 

 this superficial tissue, which is rarely preserved, is succeeded by 

 a hypodermal region consisting of parenchymatous tissue and 

 numerous vertical groups of narrow thick-walled fibres arranged 

 as radial plates or circular, oval, or reniform strands (the 

 Sparganum type of cortex). In the hypoderm as in the ground- 

 tissue generally secretory canals, often accompanied by stereome,. 

 are a characteristic feature. The vascular system is represented by 

 a considerable number of collateral bundles scattered through 

 the ground- tissue and especially abundant in the outer region: 

 the bundles sometimes assume a more or less regular disposition 

 in concentric circles. Each bundle consists of a small group of 

 xylem tracheids, for the most part spiral or scalariform, though 

 reticulately pitted elements are by no means rare, with a single 

 protoxylem group on the outer face next the phloem (fig. 420, B, C). 

 As a rule the xylem is wholly centripetal, but occasionally the 

 exarch structure becomes mesarch by the occurrence of a few 

 centrifugal tracheids. The phloem, rarely preserved (fig. 420, B), 

 consists of narrow sieve-tubes with parenchyma, and the bundle 

 as a whole is often partially enclosed by a sheath of fibres. 



1 Renault (76) B. p. 7. 



2 Grand'Eury (77) A. p. 122 ; (90) A. p. 287. 



3 Seward (93). 4 Penhallow (97). 



