412 Mas fen . — The Structure of Mesoxylon Sutcliffti [Scott). 
only parts yet known), it probably stands somewhat nearer to the Pterido- 
sperms than Mesoxylon does, although well on the way towards the typically 
Gy mnospermous family of the Cordaiteae. A feature of the Cordaiteae 
and of the plexus of form connecting this family with the Pteridosperms 
is the double leaf-trace shared at the present day by Ginkgo. Recent 
work on the occurrence of the double leaf-trace in Gymnosperms and in 
angiospermous seedlings , 1 as well as its common occurrence in so many 
Palaeozoic plants, suggests that it may have considerable taxonomic im- 
portance. 
The fusion of the twin-bundles of the leaf-trace which in Lyginoden- 
dron and Calarnopitys takes place in the pericycle, is pushed further back in 
the higher forms which lead on to the Cordaiteae. In most of these forms 
(. Poroxylon , Dadoxylon Spencer i, Mesoxylon ) fusion does not take place until 
after the perimedullary position has been reached and until the trace has 
penetrated a considerable distance down the stem, the centripetal xylem 
persisting until after fusion has been effected. In one form of Mesoxylon , 
however, viz. M. Lomctxii , the strongly converging twin-bundles fuse into 
one immediately on reaching the pith . 2 As Dr. Scott has pointed out, the 
division of the trace extends, on the whole, lower down the stem in the 
later forms . 3 
By the study of Mesoxylon and the other more ancient Cordaitean 
stems we are able to trace some of the stages in the gradual extinction of 
the centripetal wood of their Cryptogamic ancestors, a process which 
appears to have been completed in the true Cordaites. The endarch struc- 
ture thus reached in Cordaites has persisted as a characteristic feature in the 
anatomy of the stems of the higher plants. 
In conclusion, I wish to tender my grateful thanks to Dr. D. H. Scott, 
M.A., F.R.S. After the preparation of our joint preliminary note on 
Mesoxylon it had been intended to publish a full description of all the 
species under our joint names. As this work could not have been com- 
pleted for some considerable time, Dr. Scott very generously suggested that 
I should publish a detailed account of M. Sutcliffii first. Throughout the 
progress of the work I have had the invaluable help of Dr. Scott’s sugges- 
tions and criticism. I also wish to thank Mr. L. A. Boodle, F.L.S., who 
has kindly allowed the work to be done at the Jodrell Laboratory, Kew, 
and who also supplied some of the photographs used in the plates. The 
other photographs are by Mr. W. Tams. 
All the specimens were discovered by Mr. James Lomax in the Shore 
material, and the sections cut by him. 
1 Miss E. N. Thomas, A Theory of the Double Leaf-trace, founded on Seedling-structure. New 
Phytologist, vol. vi, 1907, p. 77. 
2 Scott and Maslen, loc. cit., p. 239. 
3 Studies, 2nd Edition, p» 65 2. 
