300 
PROFESSOR W. C. WILLIAMSON ON THE ORGANIZATION 
accepts on Corda’s authority) one of his distinctions between Diploxylon and Sigillaria ; 
but this distinction must now be abandoned as non-existent. Witham had described 
the large openings represented in my figures 12 & 13, in, Plate XLII. of the present 
memoir as medullary rays. Professor King, on the other hand, correctly discerned that 
these openings transmitted foliar bundles, also recording his conviction that the vacant 
spaces surrounding the bundles had probably contained cellular tissue, which I have now 
proved to be the case. 
But, relying upon Lindley’s declaration that no vascular tissue was ever found in a 
medullary ray, he denied the correctness of Witham’ s application of the term to the 
spaces in question. Mr. Binney, referring to this subject, does not speak very definitely. 
He says that his specimen “distinctly confirms Witham’s opinion as to the occurrence 
of medullary rays or bundles dividing the woody cylinder”*'; but he does not define what 
he means by bundles. At p. 600 of the same memoir he again speaks of “ medullary 
rays or bundles of barred vessels,” from which I infer that bundles of vessels are also 
referred to in the previous sentence. So far as I can ascertain, none of those observers 
who preceded me have distinctly recognized the true secondary medullary rays described 
both in this memoir and in the preceding one. 
Brongniart, Corda, and King agree in considering the Diploxylons to be Gymno- 
spermous Exogens, associating them in that great group with the true Sigillarise. 
I think the facts now published finally settle this primary question. It being admitted 
by all authors that the Lepidodendra are Cryptogams, the Diploxylons can no longer 
be regarded by any one as Gymnospermous Exogens ; and as the close identity of Brong- 
niart’s Sigillaria elegans with Diploxylon is equally obvious, we must accept the entire 
group as Lycopodiaceous. Dr. Dawson, in his recent memoir on Sigillaria f , arrives at 
a different conclusion ; but whatever may be the case with Transatlantic specimens, there 
is not the slightest room for doubt about our European ones : they are all modifications 
of the Lepidodendroid type. The distinction drawn by Brongniart between the Sigil- 
larice which have medullary rays and the Lepidodendra which have not, I have now shown 
to be merely due to difference of age. In its young state the Burntisland Diploxylon 
is an ordinary form of Lepidodendron. As it develops it passes through successive stages 
of growth, all of which appear to be more or less permanently represented amongst other 
matured Lepidodendra, though within what limits has yet to be ascertained, since, as I 
have already suggested, some of the forms described in my last memoir may be parts of the 
same plant at different ages, though in several of the examples there described this is 
certainly not the case. Long before attaining to the dimensions and stage of growth in 
ria, identical in every other respect with S.Jlcoides, but in which the medullary rays are similarly composed of 
scalariform cells. Remembering the fact that a Diploxylon from the same locality, which I described under 
Cokda’s name of D . cycadeoides, possessed the same features, the question arises, how far may these similarly 
constructed plants have borne the mutual relations of root and stem ? 
Loo. cit. p. 583. 
t Quarterly Journal of the Geological Society, May 1871. 
