[Illustration: Fig. 437. _Rhexoxylon africanum_. Transverse section of stem showing peripheral steles and central (_c_) stele. (After Bancroft; × 2.)] =RHEXOXYLON=. Bancroft. _Rhexoxylon africanum_ Bancroft. The genus _Rhexoxylon_ was instituted for a new type of stem represented by a single incomplete specimen from the Karroo series of South Africa: its precise geological horizon is not known but it may be referred provisionally to the lower or Palaeozoic portion of the series. Though our knowledge of the morphological features of the type-species is far from complete owing in part to the method of preservation of the specimen and in part to the destruction of the outer portion of the vascular tissue and the whole of the cortex, Miss Bancroft’s careful description[407] demonstrates the existence of characters which justify the employment of a new generic name. _Rhexoxylon_ is more nearly related to the Medulloseae than to any other group and is particularly interesting as the first recorded example of this group from the Southern Hemisphere. Fig. 437 shows a transverse section (7 × 5 cm.) of the stem. The ground-tissue consists of fairly large-celled parenchyma with sclerous nests and a few bands of periderm. At the periphery of the stem are radially disposed groups of vascular tissue varying in size and to some extent in shape. Unfortunately the stem is incomplete and it is impossible to say how much vascular or other tissue originally existed beyond the present corroded edge. The vascular groups, or steles as they may legitimately be called, follow a vertical course through the length of the block (6·5 cm.) and afford only slight evidence of branching or anastomosing. A close examination of the steles shows that they consist of portions of two series, an inner and outer set; there is also a curved vascular band in the central ground-tissue (fig. 437, _c_) and some isolated and scattered patches of vascular elements. Each stele of the inner series is made up of two parts, an outer smaller and normally orientated group of secondary xylem and a larger inversely orientated inner group of identical structure. A single stele of the inner series is shown in fig. 438, B, C; the larger inner portion consists of slightly divergent rows of tracheids and uniseriate medullary rays and is separated from the smaller portion by a narrow space, _a_, occupied by crushed tissue which may correspond to the ‘partial pith’ or primary xylem of a Medullosan stele. The two groups of xylem are no doubt the products of two cambium arcs, the protoxylem of each group being situated on the flat inner face. The cambium and phloem are represented only by crushed brown cells on the curved outer edge of the xylem. The separate individuality of the two portions of each stele is indicated not only by the presence of the ‘partial pith’ but by the discontinuity of the tissue at the ends of the narrow space. The tracheids seen at _b_, fig.