148 



MEDULLOSEAE 



[CEL 



compared with the steles of Medullosa Solmsi (fig. 416, L) but 

 those of RTiexoxylon differ in the lack of continuity of the secondary 

 xylem round the narrow band of crushed primary xylem. The 

 other steles of the inner ring exhibit the same dual nature though 

 with local modifications. In the stele seen in fig. 438, B, there 

 is a close approach to a continuous cylinder of secondary xylem 

 especially on the right-hand side. External to the inner series are 

 several portions of normally orientated secondary xylem-groups 

 (fig. 437) : these probably represent a second series of steles 

 separated from the inner series by a narrow crushed arc of tissue 

 on which the protoxylem strands of the outer groups abut. The 

 xylem of the outer steles agrees in its normal orientation with 



FIG. 438. Rhexoxylon africanum. A, radial wall of tracheid; B, C, peripheral 

 steles; a, primary portion of stele; b, lateral strand. (After Bancroft.) 



the outer and smaller part of the inner steles and, as there is no 

 accompanying group of inversely orientated xylem corresponding 

 to the larger mass of secondary xylem of the inner series, the 

 outer strands are designated partial steles. The central stele 

 consists of two curved irregular bands composed of vertically 

 and obliquely running tracheids: the central part of this stele 

 consists of crushed tissue that probably represents primary xylem 

 like that between the two parts of each of the peripheral steles. 

 Rhexoxylon differs from the usual Medullosan type in the 

 structure of the secondary xylem which is composed of tracheids 

 with an Araucarian form of pitting : there are usually two alternate 

 rows of contiguous pits (fig. 438, A) and occasionally one or three 

 rows. The medullary rays are uniseriate and 3 to 15 cells in depth, 

 a feature characteristic of coniferous wood and not of the wood 

 of the Medulloseae. 



