T]ie Fossil Plants of the Coal Measures. 1 1 3 



twig increases. Even in its youngest state the cortical cells 

 are remarkable for their robustness, and those of the medulla 

 soon exhibit a similar aspect. In the early growths the 

 primary tracheal cylinder is composed of a single ring of 

 conspicuously large tracheids, surrounded by a limited 

 number of very small peripheral ones ; but in their most 

 advanced state of growth two irregular rows of these more 

 conspicuous vessels exist, also surrounded by smaller peri- 

 pheral ones. This prominence of the small number of older 

 tracheids constitutes one of the characteristic features 

 of the type. Another such feature is presented by the indi- 

 vidual tracheids as seen in longitudinal sections. The 

 transversed lignified bars on their walls are connected by 

 numerous delicate longitudinal threads, like those seen in 

 the tracheids of Lepidodendron Wunschiamiin and L. Har- 

 conrtii, a distinguishing feature of this diminutive form 

 which separates it from all others with which it might be 

 confounded. 



These twigs dichotomise at a very early stage of growth. 

 I have but the one example figured in which a well- 

 developed secondary xylem strand exists. No traces of 

 leaves have been discovered on this plant. 



Youngest Twigs. 



Primary Tracheal Strand Devoid of Medulla. 

 Q. — p. 197, Figs. 7 and 8, C.N. 408. 



Cortex, Outer. 



Q.— Fig. 7, C.N. 408. 



More Advanced Growth. Primary Tracheal Cylinder and Medulla. 

 Transverse. 



Q. — p. 197, Fig. 9, C.N. 406. See also 405 A. Fig. 10, C.N. 416C 

 Fig. II, C.N. 405. Fig, 12, C,N, 413. 



Longitudinal. 



Q. — p. 197, Fig. 13, C.N. 414. See also C.N. 415. 



Cortex. 



Inner Tratisverse. 



Q. — p. 197, Fig. 12, C.N. 413. See also C.N. 411. 



Inner Longitudinal, C.N. 415- 



Q.— p. 197, Fig. 13, C.N. 414. 



