XXX] RHEXOXYLON 147 



of the vascular tissue and the whole of the cortex, Miss Bancroft's 

 careful description 1 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 (7x5 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 seiies 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 discon- 

 tinuity of the tissue at the ends of the narrow space. The tracheids 

 seen at 6, fig. C, are in oblique longitudinal section and are probably 

 being detached to form a leaf-trace. This type of stele may be 



1 Bancroft (13). 



102 



