44 Dr. W. C. Williamson on 



the number of the tracheids, there are certainly more 

 than 700. 



We next reach a branch of still larger dimensions 

 (C.N. 434). Its Primary Xylem has now a diameter of 

 5»5 mm., composed of about 729 tracheids, but which in 

 addition encloses a medulla, having a diameter of nearly 

 2'5 mm. This section is represented in Memoir X., Fig. 3, 

 and on a further enlarged representation of it is the primary 

 cylinder given in Fig. 4. In all the sections described thus 

 far the Primary Xylem gives off numerous leaf-traces, as 

 shown in the enlarged figures of Memoir X., Figs. 2 and 4. 



In section C.N. 456A, only the inner portion of the black 

 mass of outer bark exists, the more external zone of pro- 

 senchymatous periderm having disappeared. Yet we find 

 that what remains of the cortex has a circumference of fully 

 twenty-one centimetres. The diameter of the Primary 

 Xylem has reached 97 mm. and the component tracheids 

 have now reached 2,225 in number. In C.N. 452 this 

 corresponding cylinder has enlarged to 22mm. and the 

 tracheids increased to 6,900. The Secondary Xylem reached 

 a diameter of 69mm. But a still more conspicuous example 

 of what appears to have been a primary stem (C.N. 350) has 

 advanced yet further. What remains of the cortex is still 

 23 centimetres in diameter. Its Primary Xylem cylinder 

 is 36*5 mm. in diameter and its component tracheids about 

 19,220 in number. In addition this is the smallest stem- 

 section in which we find a trace of a secondary xylem. It 

 is a very thin ring investing the primary cylinder, having 

 only a thickness of 4 mm. on one side and but 1 mm. on the 

 opposite one of the ring. These conditions demonstrate 

 that in this type of Lepidodendron the upper branches 

 attained to very large dimensions before the secondary 

 growth of xylem began to be developed. 



The magnitude of the central medulla is remarkable, 

 being fully 24 mm. in diameter. The next section, C.N. 



