XXX] MEDULLOSA 91 



of two of the petioles is continuous with that of the stem, while 

 that of the third leaf-stalk is cut through near its separation from 

 the stem and its adaxial face is already defined by a hypodermal 

 band of stereome, d. The surface of the stem is characterised 

 by fine longitudinal ribs caused by the slightly projecting stereome 

 in the outer cortex, and from the narrow furrows between the 

 leaf-bases adventitious roots emerge in vertical series. The 

 position of an interfoliar furrow is shown by a small arrow 

 in fig. 416, A. There are three steles, 2-3 cm. x 6-10 mm. in 

 diameter: each agrees very closely in structure with the single 

 stele of Heterangium. Medullosa anglica may be described as 

 a polystelic Heterangium and as having the same relation to 

 Heterangium as regards the stelar system as Primula auricula 

 bears to the monostelic Primula. The central core of the stele 

 (the black patches in the diagram, fig. 416, A) consists of an 

 anastomosing system of tracheal groups embedded in an irregular 

 parenchymatous reticulum. The large primary tracheids reach 

 a diameter of 150 /x and have multiseriate pitting : at the periphery 

 of the primary xylem there is a more definite grouping of tracheids 

 as in Heterangium, and the slightly internal (mesarch) protoxylem 

 elements are associated with scalariform and densely spiral 

 tracheids (fig. 416, B, C) narrower than the more internal reticulate 

 elements. The secondary xylem is manoxylic as in Cycads, 

 tracheids in 2-4 radial series alternating with medullary rays 

 1-3 cells broad and usually of considerable depth (fig. 416, B). 

 The principal rays are continuous with the parenchymatous matrix 

 of the central core. Thick-walled tubular elements, no doubt of 

 the nature of sieve-tubes, form a conspicuous feature in the 

 phloem. 



The three steles occasionally divide and fuse with one another. 

 The tissue between the steles is crushed and disorganised and in 

 the living plant was probably small in amount. In the imper- 

 fectly preserved inner cortical region there is a sinuous band of 

 secondary parenchyma (periderm; fig. 416, A, c) developed from 

 a deep-seated phellogen; in older stems this formed the super- 

 ficial tissue after the fall of the leaves. There is no definite 

 boundary between the cortex of the stem and the petiole-bases 

 except when the hypoderm cuts across the cortex preparatory to 



