172 PROF. W. C. WILLIAMSON ON THE STRUCTURE 



Hooker are disposed to regard the type described by 

 Binney and Unger as Equisetaceous. On the other hand, 

 though M. Brongniart believes in an Equisetaceous form 

 of Calami te, he does not regard the above examples as be- 

 longing to it. He considers that they are Calamodendra, 

 which he places amongst the Gymnospermous Exogens. 

 Are we yet in a position, in the face of these discrepant 

 opinions, to arrive at a conclusion on the moot point ? 

 The importance now attached to the doctrine of evolution 

 gives significance to the question, and renders an answer 

 desirable. 



The inferiority of the cellular to the vascular plants is 

 obvious and admitted. When we ascend to the vascular 

 Cryptogams, we find that they retain indications of their 

 natural alliance with the cellular forms, in having the 

 vascular elements largely intermingled with cellular ones ; 

 whilst in the Gymnospermous Exogens the purely cellular 

 element is almost eliminated from their woody layers, 

 being only represented there by the medullary rays. 

 Viewed in this aspect, Mr. Binney's Calamodendron 

 appears to approximate to the recent Acrogens, in whose 

 stems cellular and prosenchymatous tissues are abundant, 

 combined with vessels of a scalariform type, but which, so 

 far as I know, exhibit neither reticulated vessels nor me- 

 dullary rays. But when we ascend to the Gymnospermous 

 Exogens, we find the cellular element disappearing, whilst 

 vascular tissues present themselves, chiefly in a reticulated^, 

 spiral, scalariform, or glandular form. In my plant, whilst 

 we have abundance of muriform medullary rays, we have 

 few if any transversely barred vessels ; every duct of which 

 the structure can be traced is distinctly reticulated, 

 whilst the prosenchymatous cells, as we have seen, assume 



* I may observe that a small form of reticulated pleurenchyma, ap- 

 parently almost identical with the reticulated vessels of my plants, enters 

 largely into the woody zone of the living Araucaria imbricata. 



