MEDULLOSEAE 449 



stem, which must have been a small specimen — probably 

 from a young plant. 



The main feature of the anatomy, in which Sutcliffia 

 differs from any Medullosean stem previously known, 

 is the presence of a single central stele, of large size 

 (measuring 4.7 x 1.8 cm. in the section figured, Fig. 

 168). There is no pith, and the wood has the same 

 structure as in a stele of Medullosa anglica, except that 

 in Sutcliffia the protoxylem-groups are peripheral, the 

 xylem thus being exarch. A zone of phloem, in 

 which strands of sieve - tubes can be recognised, 

 surrounds the wood. In the stem investigated (the 

 only specimen as yet discovered) secondary growth 

 was just beginning. 



From the main stele, large, irregular strands, the 

 meristeles or subsidiary steles, were detached at intervals, 

 giving a remarkable and unique appearance to the 

 transverse sections (see Fig. 168, a, B, and S). The 

 meristeles break up into smaller strands, but the main 

 branches of adjacent meristeles often fuse with one 

 another. Thus the large mass B, in Fig. 168, is the 

 product of such a fusion, as shown by the comparison 

 of serial sections. The strands derived from the further 

 subdivision of the meristeles or their fused branches 

 (Fig. 168, a, /3', B') ultimately constitute the numerous 

 bundles of the leaf- trace (Fig. 168, d.r., v.b.). The 

 vascular strands in the leaf-bases and petioles always, 

 however, retain a concentric structure, and their xylem 

 contains parenchyma, so that they preserve a more 

 stele-like character than the corresponding bundles in 

 a Medullosa. The structure of the petiole agrees in 

 all essentials, though not in every detail, with that of 



