Ke 2824 7 
of the Osmundacee. By A. C/ Srwak, F.R.S., F.L.S., University Lecturer 
V. The Anatomy of Todea, with d E Geological History and. Affinities 
in Botany, and SIBILLE O. Fonb, Bathurst Student, Newnham College, Cambridge. 
( Plates 27-30.) 
Read 17th April, 1902. 
INTRODUCTION. 
‘THE Osmundacew have, on various grounds, been frequently referred to as occupying 
a somewhat isolated position among existing Ferns. Experience bas shown that isolation 
is usually associated with a past history which reveals a greater vigour in former ages, 
as demonstrated by an abundance of fossil representatives widely distributed both in 
space and time. There are not a few surviving species of plants that remain as 
examples of blindly ending lines of evolutionary progress; or as relics of structural 
types, which have survived as evidence of a stage in phylogenetic development, from 
which have been evolved other styles of plant-architecture destined to assume a leading 
róle. It is only from a few scattered and solitary examples that we obtain glimpses of 
pre-existing types or transitional styles, which enable us to trace to their origin the 
prevailing structural plans as illustrated by existing classes and families of plants. 
The former prominence of the Osmundace:x in the vegetation of the world is demon- 
strated by an examination of Mesozoic floras from different regions. On the other hand, 
opinions differ as to the interpretation of the anatomical structure of the stem as an 
isolated type of plant-anatomy among existing Ferns. De Bary described Osmunda as 
affording an example, in the structure of its stem, of the occurrence of the ** Dicoty- 
ledonous type” among the Filices *. Haberlandt} sees in the Osmundaceous central 
cylinder a transitional stage, which has persisted as an index of the course of events 
whieh marked the gradual evolution from a solid concentric stele (protostele) to the 
type of stele represented by Dicotyledons and Conifers. He interprets the continuity 
of the phloem-zone in Osmunda as a remnant of an earlier form of stelar structure in 
which the xylem also existed as a complete cylinder; in the course of evolution the 
woody portion of the vascular tissue became separated into distinet strands by outward 
radial extensions of a central pith, and a further encroachment of the medullary tissue 
caused a similar division of the phloem-zone, resulting in a ring of collateral bundles 
separated by medullary rays. Another interpretation of the Osmundaceous plan has 
i * De Bary (1884), p. 280. + Haberlandt (1896), p. 312. 
SECOND SERIES.—BOTANY, VOL. VI. 2M 
