no Dr. W. C. Williamson on 



C & E) ; we have the primary xylem strand composed of a 

 cluster of barred tracheae, but in the centre of which there 

 may possibly be a few medullary cells ; in my smallest 

 ordinary twig (C.N, 1596 G) we already find a minute but 

 well-defined medulla. The tracheal of the primary cylinder, 

 even in these young states, show the delicate vertical threads 

 connecting the transverse bars already mentioned as occur- 

 ring in the tracheae of L. Wnnschianwn. In all these 

 very young twigs there is but little differentiation of 

 the zones of the cortex ; in rather older ones we find 

 an innermost cortex composed of delicate thin-walled 

 parenchyma (C.N. 38oa-38i). The inner portion of the 

 middle cortex has entirely disappeared. The thickness of 

 the cell-walls of the outer-middle cortex increases as we 

 approach the prosenchymatous zone. This zone commences 

 as a segment only of a circle, but is soon converted by 

 lateral extension into a perfect cylinder. In transverse 

 sections of the more advanced stem this zone projects out- 

 wards in numerous acute vertically elongated prominences 

 through which leaf-traces emerge to reach the leaves ; the 

 bases of these latter organs rest, as usual, upon a thin layer 

 of cortical parenchyma. Varied sections of the leaves 

 exhibit the characteristic organs of the Lepidodendroid ty^Q, 

 having the leaf-trace, the double parichnos, and the adenoid 

 organ. 



What seem to be Halonial tubercles exist in these young 

 growths, supplied with the large tracheal bundles devoid of 

 medulla, so characteristic of Halonial and Ulodendroid 

 appendages. 



Medulla. 

 Transverse. 



BB.— p. 3, Fig. la, C.N. 1596A. 

 p. 7, Fig. 3a, C.N. 380. 

 p. 7, Fig. 6a, C.N. 380a. 



Longitudinal. 



BB.— Fig. 26a C.N. 380b. 



