76 



PTERIDOSPERMEAE 



[CH. 



with dark contents in the stem-tissues probably represent secretory 



sacs. 



The leaf-traces on leaving the stele pursue a very gradually 

 ascending course to the petiole; they retain their collateral 

 structure in the pericycle and cortex and have no secondary 

 xylem, but become concentric as they enter the base of a leaf. 



FIG. 412. Heterangium Grievii. Oblique longitudinal section of stele and part 

 of cortex; p, pericycle. x 3. (Kidston Coll. 529.) 



Before passing to the description of the leaves, the more striking 

 features in the stem may be summarised with reference to the 

 diagrammatic sketches shown in fig. 411. Fig. 411, A, represents 

 a section of Heterangium Grievii approximately 2 cm. in its 

 maximum diameter ; at the periphery of the primary xylem, x 1 , 

 and close to its outer margin are several protoxylem groups, 

 not shown in the drawing, each of which marks the position of 

 a mesarch trace. The zone of secondary xylem, x 2 , is interrupted 

 by the exit of leaf- traces and one of these is seen at a in fig. 415, A, 

 separated from the central primary xylem by a foliar gap filled 

 with parenchyma. The pericycle is shown at p in fig. 412 and 



