Worsdell. — The Structure and Origin of the Cycadaceae . 133 
The ovule consists of the following parts : an outer fleshy, often brightly- 
coloured envelope which is intimately fused with a thick woody shell 
internal to it, within which again is another soft-celled tissue. The vascular 
bundles supplying the ovule are divided into two distinct systems : an 
outer one traversing the external fleshy envelope, and an inner supplying 
the internal soft tissue. 
These, from the purely descriptive point of view, are the main facts 
of the structure of modern Cycads. 
I will now see what we know as to the structure of the Medulloseae , 
a group of the Pteridosperms of which specimens have been obtained from 
the Coal Measures and the Permian. We know little as to the habit of 
these plants, but some of the stems found are of considerable thickness 
and length. 
The vascular anatomy of the stem is of great interest, presenting 
a structure which combines the characters of that of Ferns and modern 
Cycads, for it exhibits on the one hand polystely and on the other well- 
developed secondary tissue in each stele. 
If we take a species, viz. Medidlosa Solmsii, which is one of the most 
Fern-like in its structure, we find, in a transverse section of the stem, 
a double ring of concentric bundles or steles surrounding a wide pith and 
surrounded in their turn by a wide cortex. The steles of the inner ring 
are somewhat smaller than those of the outer. Each individual stele in 
both rings consists of a central pith, in which primary tracheides are 
scattered, surrounded by a considerable development of secondary xylem 
and phloem. Small, concentric leaf-trace bundles occur just outside the 
outer ring of steles. In the pith are scattered very numerous, minute, 
concentric bundles, each largely formed of secondary tissue. 
In M. anglica, Scott, there are only three steles of irregular shape and 
large size, each possessing a structure identical with that just described. 
Here and there, immediately outside the steles, is seen a much smaller, 
isolated, concentric strand termed by Scott ‘ anomalous ’ in the sense of 
representing something extra to the main group of large steles. 
The leaf-trace bundles, immediately on leaving the steles, are perfectly 
concentric in structure, but very soon break up into a small group of 
collateral bundles, which still further subdivide on entering the leaf. 
The stem of M. porosa, Gotta, belongs to the type in which the cylinder 
(at any rate frequently) is composed of a continuous band or zone of 
vascular tissue, which possesses in itself precisely the structure of one of the 
concentric steles of M. anglica , Scott, or M. Solmsii^ Schenk. In this 
species the mutually-inverted parts of the secondary xylem and phloem are 
respectively equal in development. Numerous medullary bundles, of very 
much larger size than those of M. Solmsii, Schenk, occur, scattered irregu- 
larly through the pith ; many of them are imperfectly concentric in structure. 
