xv] 



LEPIDODENDRON 



149 



of the secretory zone, sc. The longitudinal section (fig. 169, D) 

 shows the cambial cells, a, next the secretory zone, sc, passing 

 internally into crushed and imperfectly preserved elongated 

 elements which are presumably miniature tracheae, and these 

 are succeeded by older and more completely lignified xylem 



Fio. 169. Lepidodendron fuliginosum. 



A, B. (Manchester UniverBity Collection. No. Q. 645 A.) 



B, C. (Manchester. No. 257.) 

 D. (Manchester. No. 6.) 



elements, x. In larger shoots the amount of secondary tissue 

 is considerably greater ; it may consist almost entirely of short- 

 celled parenchyma (fig. 168, C, from x to sc), or it may include 

 a large proportion of radially disposed and vertically elongated 

 tracheae (fig. 168, D, x\ and fig, 170, A, ar^), or it may consist of 



