of Goal-Measure Plants. 109 



type is the unusually large size and the peculiar short form of the 

 metaxylem tracheids, which differ both in their size and shape, 

 especially in the latter, from those of Heterangium, as well as in 

 their somewhat more irregular grouping. The difference in size is 

 of little importance and indeed some of the metaxylem tracheids 

 of Heterangium 1 are almost equal in dimensions to those of 

 Megaloxylon. The perimedullary exarch strands of tracheids in 

 Megaloxylon are common to the stem and leaves and consist entirely 

 of leaf-trace bundles; in Heterangium the peripheral mesarch 

 strands are " greatly in excess of the number of orthostichies which 

 appear to have existed 2 ." Williamson and Scott describe the 

 perimedullary strands in Heterangium as differing from those of 

 Lyginodendron in not being isolated groups of tissue, but as 

 forming part of a continuous mass of wood. The greater number 

 of the peripheral strands in the former genus as compared with 

 the number of orthostichies on the stem may be explained, as 

 suggested by Williamson and Scott, in three ways : — " (i) the leaf- 

 traces on joining the cylinder may have branched, or (ii) they may 

 have extended down through a very large number of internodes 

 turning aside sufficiently to make room for each other, or (iii) it 

 may be that some of the peripheral strands of the cylinder are 

 cauline and not directly continuous with the leaf-trace bundles 3 ." 

 The structure of Megaloxylon suggests another explanation ; it is 

 not improbable that the leaf-trace bundles of Heterangium in their 

 downward course became laterally extended, and in spreading gave 

 rise to a greater number of tracheal groups as described above in 

 the Binney stem 4 . 



The fairly numerous protoxylem strands and the exarch 

 structure constitute distinctive features of the leaf-trace of Me- 

 galoxylon as seen in its course through the secondary xylem (PI. 

 Vll. fig. 10) compared with the emerging leaf- traces of Lyginoden- 

 dron 5 and Heterangium. In the large undivided leaf- trace of Medul- 

 losa anglica figured by Scott in his Plate XL fig. 10 6 there is a 

 protoxylem strand in an apparently exarch position, and the 

 general appearance of the Medidlosa leaf-trace resembles that of 

 the corresponding vascular axis in Megaloxylon Scotti. In Medul- 

 losa anglica this large leaf-trace gradually breaks up into the 

 characteristic exarch bundles of the Myeloxylon type as it passes 



1 Cf. section 1619 in the Williamson Collection, British Museum; also Heteran- 

 gium bibractense, Renault (93), p. 252. 



2 Williamson and Scott (95), p. 747. 



3 Williamson and Scott (95), p. 748. 



4 My friend, Dr Scott, on seeing the sections of the new genus pointed out to 

 me the possible bearing of the structure and behaviour of the leaf-traces on 

 the question discussed in the joint memoir on Lyginodendron and Heterangium. 



5 Williamson (73), PI. xxvi. fig. 18. 



6 Scott (99). 



