. Fossil plants : for students of botany and geology . Paleobotany. 148 MEDITLLOSEAE [CH. compared with the steles of Medullosa Solmsi (fig. 416, L) but those of Rhexoxylon 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 cyhnder of secondary xylem especially on the right-hand side. External to the inner series are several portions of normally orientated secondary xylem
. Fossil plants : for students of botany and geology . Paleobotany. 148 MEDITLLOSEAE [CH. compared with the steles of Medullosa Solmsi (fig. 416, L) but those of Rhexoxylon 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 cyhnder 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. steles; A B C Rhexoxylon africanum. A, radial wall of tracheid; B, C, peripheral u, primary portion of stele; 6, 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 seriesj the outer strands are designated partial steles. The central stele consists of two curved irregular bands composed of vertically and obhquely running tracheids: the central part of this stele consists of crushed tissue that probably represents primary xylem hke that between the two parts of each of the peripheral steles. Rhexoxylon differs from the usual MeduUosan 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 Please note that these images are extracted from scanned pa
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