THE LYCOPODIALES 



253 



FIG. 178. Diagram of the lower region of the stem 

 in Phylloglossum (after Bertrand). 



axis the fibrovascular tissues become so much reduced that internal 

 phloem is no longer developed, and the continuity of the cylinder 

 is interrupted by gaps which are not related to organs, but merely 

 indicate the incom- 

 plete development 

 of the xylem . 

 Where a trace is 

 given off, as is 

 shown in Fig. 179, 

 it takes its origin 

 opposite a strand 

 and does not sub- 

 tend an interval be- 

 tween the bundles, 

 clearly showing 

 the lycopsid condition, even in the state of stelar reduction pre- 

 sented in the evan- 

 escent aerial axis. 



The Psilotaceae are 

 anatomically distin- 

 guished from the Lyco- 

 podiaceae by the 

 absence of true roots. 

 Here the aerial stem, 

 unless it be of very 

 small size, is siphono- 

 stelic in its organiza- 

 tion. A thick-walled 

 medulla is often pres- 

 ent, but no internal 

 phloem has been ob- 

 served. The organiza- 

 tion of the conducting 

 tissues is radial and exarch; the leaf traces, as in Lycopodium, take 

 their origin from the angles of the stele. In smaller aerial shoots and 

 in the subterranean ones the central cylinder is usually protostelic. 

 Sometimes gaps are present in the walls of the tubular cylinder of 



FIG. 179. Diagram of exit of leaf traces in the 

 aerial stem of Phylloglossum (after Bertrand). 



