22 
PROFESSOR W. C. WILLIAMSON ON THE ORGANIZATION 
the vascular axis of the fruit Calamostachys JBinneana (Phil. Trans. 1874, p. 59 et seq., 
figs. 33-39), of the stem of Asterophyllites, and of the medullary axis of the Lepido- 
dendroid plants described and figured in my second memoir on the Plants of the Coal- 
measures ( loc . cit. 1872, Plate 24. figs. 1 & 2). I have already given my reasons for 
regarding all these three groups of plants as Lycopodiaceous ; and it is my conviction 
that the plant now described belongs to the same natural family. At the same time I 
need scarcely remark that it differs from all known recent Lycopodiaceee. Several of the 
living Lycopods exhibit a defined epidermal layer, investing a cortical parenchyma com- 
posed of large cells full of chlorophyl. Lycopodium alpinum exhibits this condition ; 
and in Selaginella Martensii a similar epiderm, composed of colourless cells, becomes 
very conspicuous at the margins of the leaflets, where it projects in comparatively large 
tooth-like, almost unicellular, hairs or spines. But none of these plants exhibit any thing 
that exactly resembles even the primary central vascular bundle of the fossil form. The 
nearest approach to it is perhaps to be seen in Psilotum triquetrum ; but we have 
nothing resembling the exogenous growth of the six primary vascular wedges in any 
recent Lycopodiaceous stem. In the peculiar structure of its entire vascular axis this 
plant stands alone amongst the varied types found in the Carboniferous beds. In the 
structure of its central vascular axis, especially as seen in the young twigs (figs. 24 & 25), 
it bears a considerable resemblance to the Lycopodium Penaultii described by Professor 
Bexault *; but in the other details of their respective structures I can find few, if any? 
resemblances between my plant and that of my distinguished friend. In the L. Penaultii 
its discoverer has as yet found no trace of the exogenous growth which is so charac- 
teristic of my example. 
That the plant is a Cryptogam I think there can be no doubt. It presents no features 
that indicate a Dicotyledon. All its vessels are mere modifications of the spiral and 
barred tissue which constitute so universal a feature in the stems of the other Carboni- 
ferous Cryptogams. The sclerenchymatous prosenchyma of the woody rings of the 
Dicotyledons is entirely wanting to its woody zone. It possesses this feature in common 
with the Catamites , Lepidodendrce , Asterophyllites, and other Carboniferous Cryptogams 
which I have already described. 
In one respect the plant under consideration is one of the most remarkable of any that I 
have yet seen. Though the largest stem that I have found does not exceed one eighth of 
an inch in diameter, which is the thickness of that represented in fig. 23, yet its exogenous 
development is complete. In criticising my views in an early stage of my inquiries, my 
friend Professor Thisleton Dyer expressed his conviction that these supposed exogenous 
growths were mere questions of size, and had no physiological meaning. But such cannot 
possibly be the case here. The axis of nearly every known living Lycopod has a diameter 
equal to that of the fossil before us ; yet we find the exogenous structure of the latter 
as conspicuously developed as in the largest of the Lepidodendroid stems. Whatever 
* “ Etude de quelques vegetaux silieifie's des environs d’Autun,” Annales des Sciences Naturelles, 5 e serie, 
Lot., t. xii. (Cahier no. 3), pp. 82-85. 
