100 Proceedings of the Royal Physical Society. 



der of small barred vessels, arranged in parallel series, radiat- 

 ing from the medulla outwards. These represent the ligneous 

 zone. The innermost ones are exceedingly minute, and 

 though they increase in size as we proceed outwards, they 

 rarely exceed "016 in diameter, the great majority of them 

 being much smaller. It is from the innermost surface of this 

 cylinder that the vascular bundles are given off to the leaves, 

 a point of importance in determining the homologous relation- 

 ships of the various portions of the Lepidodendroid plants. 

 The radiating arrangement of these vessels suggests, as the 

 quotation already made from the writings of M. Brongniart 

 points out, an exogenous mode of growth, a conclusion fully 

 borne out by the facts yet to be mentioned ; small cells, 

 arranged in single or double vertical rows, pass outwards, like 

 medullary rays, between these vessels. 



" The tissue immediately surrounding the ligneous zone 

 has almost always disappeared from the specimens of this 

 plant, its pla-ce being represented by an almost vacant space ; 

 but there are indications, as Mr Carruthers has correctly 

 pointed out, that it has been a delicate form of parenchyma. 



" In the present example almost every trace has disap- 

 peared, save a narrow ring of disorganised carbonaceous mat- 

 ter at some little distance from the ligneous zone. The space 

 within this tissue represents the innermost portion of what I 

 regard as the cortical layer. . . . 



" We now come to the middle bark, a dense, well-preserved 

 layer of thick walled parenchyma, gradually passing into 

 prosenchyma at its outer margin. ... As the paren- 

 chyma of this middle bark becomes converted into prosen- 

 chyma at its outer portion, its cells become elongated ver- 

 tically, and at last pass rapidly into the almost vascular form 

 of prosenchyma, constituting the bast layer of the outer bark. 

 . . . Towards the outermost portion of this tubular pro- 

 senchyma we find, in these fossils, a tendency to split verti- 

 cally, and to the consequent detachment of the epidermal 

 layer. The innermost portion of this detached layer consists of 

 tubes precisely similar in every respect to those of the outer 

 bark, but which again change rapidly as we proceed outwards, 

 first into the prosenchymatous form seen in the middle bark. 



