OSMUNDACEAE 539 



X 



an opinion opposed to a theory of reduction is to be found in the ontogeny 

 of the living plants : for in the seedlings there is at first a protostele without 

 internal complications, which expands later, and becomes medullated ; but 

 at first the passing out of a leaf-trace does not necessarily interrupt the 

 continuity of the xylem-ring : leaf-gaps aje not found till later. 1 Thus 

 the ontogeny suggests a progressive evolution of the complex structure 

 from the protostele. 



But still more cogent evidence is derived from the study of the structure 

 seen in the related fossils, examined successively according to their strati- 

 graphical succession. This work has lately been carried out by Kidston 

 and Gwynne-Vaughan, and the demonstration is a very convincing one. 2 

 If the present Osmundaceous structure be reduced, the fossil correlatives 

 should show a progressively more complex structure as they are followed 

 to earlier strata, but the reverse is found to be the general trend. Five 

 salient stages of complexity of the stele are involved in the series recognised 

 by Kidston and Gwynne-Vaughan: they are these: (i) the condition with 

 interrupted xylem-ring, and internal endodermis and phloem; (2) an 

 interrupted xylem-ring surrounds pith only ; (3) a continuous xylem-ring 

 surrounds the pith ; (4) a solid xylem is present, without pith, but 

 heterogeneous in structure (?) ; (5) a solid homogeneous xylem. 



In the modern Osmundaceae the usual condition is (2), but with 

 indications of (i) in O. cinnamomea, and less clearly in T. hymenophylloides. 

 Among the fossils Osmundites Dowkeri, Carr, from the Eocene, shows 

 the condition (2). Osmundites Skidigatensis, Penhallow, from the 

 Cretaceous, shows internal phloem, and is in fact the most complex 

 Osmundaceous structure known. If no other fossils were available than 

 this, there would appear to be some support for a reduction theory; but 

 other fossils preclude this conclusion : 3 thus Osmundites Chemnitziensis, 

 Unger, from the Tertiary Quartz of Hungary, shows the condition (2). 

 From the Jurassic comes Osmundites Gibbeana, Kidston and Gwynne- 

 Vaughan, which shows the structure of type (2), but with narrow leaf-gaps : 

 also Osmundites Dunlopi, Kidston and Gwynne-Vaughan, with a continuous 

 ring of xylem surrounding a central pith (3). From the Permian of Russia 

 Chelepteris grarilis, Eichwald, which shows type (3), with continuous 

 xylem-ring : also Chelepteris Zalesskii, Kidston and Gwynne-Vaughan, 

 which appears to conform to type (4), showing a protostelic state, but with 

 the central region of the xylem differentiated from the peripheral. This 

 condition approaches very near to type (5), with solid homogeneous proto- 

 stele, a state which is seen in Grammatopteris, from the Permian of Autun. 

 It has already been remarked above (p. 499) that in general habit and 



1 Seward and Ford, /.<-., p. 241. 



2 Kidston and Gwynne-Vaughan, Trans. Roy. Soc., Edin., 1907, vol. xlv., p. 759. 



3 An example such as this, leading to a possible conclusion which wider knowledge 

 of the fossils shows to be erroneous, exemplifies one of the many dangers of argument 

 from fossil evidence. 



