6 4 
Scott — On a Palaeozoic Fern , the 
system of Osmundaceae ; probably, however, the relation is too indirect for 
any such detailed comparison to be admissible. 
Finally, some reference must be made to Dr. Bertrand’s theory of the 
general course of evolution of the Zygopterideae. 
Asterochlaena , though complex in the structure of the stem, has 
a simple form of petiolar bundle, the xylem forming a curved band, concave 
towards the stem, with a peripheral loop, containing the protoxylem, near 
each end, and slightly towards the abaxial surface. The only simpler foliar 
bundle of this group is that of Clepsydropsis , which is straight, and is thus 
symmetrical about two planes, while the bundle in Asterochlaena is only 
symmetrical about one plane (the radial). 
Dr. Bertrand (’ 08 ) believes that Clepsydropsis is the petiole of Cladoxylon , 
though the two have not been found in connexion, and the proof is still 
incomplete. Cladoxylon has a very peculiar polystelic structure, the steles 
having, on the whole, a radiating arrangement. Dr. Bertrand holds that the 
xylem-star of Asterochlaena was probably derived by condensation from the 
dispersed xylem-system of Cladoxylon . A greater condensation, he adds, 
would clearly give rise to a solid or annular mass of wood with a circular 
contour (’ll, p. 54). Both in Diplolabis Romeri and Ankyropteris corrugata 
he derives the circular xylem from a stellate xylem by condensation. 
He appears, however, to regard the Ankyropteris Grayi type not as an 
intermediate stage of condensation, but rather as a specially high form of 
differentiation (see p. 56). He supposes that phenomena of condensation 
have occurred simultaneously in all the parallel series of the family. The 
more highly differentiated members may be expected to show — 
(1) a solid xylem, 
(2) a continuous xylem-ring, or 
(3) a xylem-ring differentiated into alternate reparatory and leaf- 
generating regions, this being the highest type of all. 
In the more primitive members of the family, on the other hand (i. e. 
those nearest to Clepsydropsis ), we shall be likely to find a stellate vascular 
mass. Dr. Bertrand enumerates Diplolabis , Tubicaulis (which has a proto- 
stele), and Ankyropteris among the advanced Zygopterideae, Asterochlaena 
and Zygopteris Kidstoni among the primitive forms. 
I do not propose to discuss this interesting theory in detail ; a better 
opportunity may be found in a future communication. It is necessary, 
however, to point out that two opposite views are held: Dr. Bertrand’s 
theory of condensation from a primitive polystelic structure, and Dr. Gordon’s 
theory (shared, I believe, by most palaeobotanists) of the gradual elaboration 
of an undifferentiated protostele. Dr. Bertrand’s view really depends on 
the truth of his attribution of the simple petiole Clepsydropsis to the complex 
stem Cladoxylon ; it will scarcely command general assent until this identi- 
fication has been placed beyond doubt. 
