OSMUNDACEAE 539 
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 are not found till later! 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.? 
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: (1) 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 (1) in O. cimnamomea, and less clearly in Z: hymenophyllotdes. 
Among the fossils Osmmundites 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:* thus Osmundites Chemnitsiensis, 
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 Dunilopi, Kidston and Gwynne-Vaughan, with a continuous 
ring of xylem surrounding a central pith (3). From the Permian of Russia 
Chelepteris gracilis, Eichwald, which shows type (3), with continuous 
xylem-ring: also Chelepieris 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 Grammatopterss, 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, Zrans. 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. 
