[Illustration: Fig. 412. _Heterangium Grievii_. Oblique longitudinal section of stele and part of cortex; _p_, pericycle. × 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 its outer boundary at _p_ in fig. 411, A. Beyond the pericycle is the broad parenchymatous cortex with leaf-traces, _lt_, and some sclerenchymatous patches, _s_. The vascular strand _a_ is passing into the base of a leaf-stalk. In the stem shown in fig. 411, B, 1·4 cm. × 7 mm., a decurrent petiole is seen at the upper end with its single vascular strand, _a_, and two sclerous nests; a similar though detached leaf-base occurs at the opposite end of the long diameter. Other leaf-traces are seen at _b_ and _c_. From the left-hand side of the stele a curved strand of tracheids is passing out to supply a root, _r_. ii. _Root_. Adventitious roots of endogenous origin are occasionally met with in _Heterangium_ stems, but we have less information as to their anatomy than in the case of _Lyginopteris_. In a specimen of _Heterangium Lomaxi_ figured by Williamson and Scott[220] three roots are seen in a vertical series growing outwards through the cortex of a stem. The roots agree generally with those of _Lyginopteris_ but the outermost cortical layers possess no special features. iii. _Leaf_.