xv] 



LEPIDODENDRON 



151 



The longitudinal section (%. 168, C) shows the outer edge of 

 the primary xylem, x, abutting on a band of dark and small- 

 celled parenchyma which passes into the broad zone of secondary 

 tissue, m, the inner region of which consists of fairly thick- 

 walled elements in radial series passing externally into the 

 thin-walled cells of the cambial region, a, on the inner edge of 

 the secretory zone, sc. This section shows also the interruption 



Fio. 171. Lepidodevdron faliginosum. (Prom sections in the Manchester 

 Museum.) 



of the secretory zone by an out-going leaf-trace. It, the lower part 

 of which, sc, is continued downwards in to the secretory zoiie. The 

 exit of a leaf-trace produces a gap in the secretory zone of the 

 stem, but not in the xylem. If we applied the term phloem to the 

 secretory zone — a course adopted by Prof F. E. Weiss and some 

 other authors, but which I do not propose to follow — we should 

 speak of a phloem foliar-gap as a characteristic feature of a 



