MEDULLOSEAE 433 



bordered, just as in the genera last mentioned. Only 

 slight remnants of the cambium are preserved, but 

 sufficient to show that it was in the normal position, 

 forming wood internally and phloem externally, with 

 reference to each stele. The phloem itself is fairly 

 preserved in places (Fig. 165, ph), and consisted of 

 strands of long tapering elements, clearly the sieve- 

 tubes, forming a network, the meshes of which were 

 occupied by the rather wide phloem-rays. In the 

 Binney specimen, described by Mr. Arber, the sieve-tubes 

 are perfectly preserved, and have conspicuous sieve- 

 plates on their lateral walls, just as in Heterangium 

 tiliaeoides} There appears to have been a pericycle 

 around each stele, but its structure is ill-preserved. 



So far, the general organisation of Medullosa anglica 

 may be roughly described as that of a polystelic 

 Heterangium. In the points which remain to be 

 described, the Heterangium type is departed from more 

 widely ; the differences are partly correlated with the 

 polystely, and partly depend on the greater size and 

 complexity of the plant as a whole. 



The leaf-trace bundles were given off, for obvious 

 reasons, only from the free, peripheral side of the steles. 

 Where a leaf-trace first becomes free from the stele, 

 it is a large bundle, to all appearance truly concentric 

 in structure (see Fig. 165, l.t). It consists of a central 

 mass of primary wood, with one or more protoxylem- 

 groups near its external margin. This is surrounded 

 by a zone of secondary wood and phloem. The 

 primary wood of the leaf- trace is continuous with the 

 outer part of that of the stele from which it springs. 



1 Arber, loc. cit. Plate xx. Eig. 4. 



