7 
Mesoxylon and an Allied Genus. i 
Without altering the generic characters of Mesoxylon it is impossible 
to include the stems in question in that genus, for the generic diagnosis con- 
tains the statement : ‘ Leaf-traces double where they leave the pith, the two 
strands uniting at a lower level ’ (Scott and Maslen, 1910, p. 237). There is 
no question of dropping this character, on which great stress has been laid 
in all descriptions of the species of Mesoxylon. It must, however, be under- 
stood that it applies essentially to the vegetative stems only. 
It thus becomes necessary to establish a new genus for the specimens 
just described, and the name I propose to adopt is Mesoxylopsis. I think 
we are logically compelled to include in the new genus the two bud-like 
shoots (of the 2598 and 3017 series) previously described. They too have 
single leaf-traces, and the evidence goes to show that, the shoots were vege- 
tative organs. The fact that they bore polydesmic leaves shows that the 
singleness of the leaf-trace was not simply an adaptation to a reduced foliar 
structure. The case for separation from Mesoxylon is in fact just about as 
strong for these buds as it is for the larger stems just described. Although, 
therefore, we cannot absolutely prove that the bud-like shoots may not have 
been peculiar specialized branches of a Mesoxylon, the presumption on present 
evidence is that they belonged to Mesoxylopsis , and I have taken account of 
their characters in drawing up the following diagnosis of the new genus : 
Mesoxylopsis, gen. nov. 
Pith probably discoid in the mature stem. 
Wood dense, with narrow, usually uniseriate medullary rays. 
Leaf- traces single where they leave the pith and pass through the 
wood, forking repeatedly in the cortex and leaf- base. 
Centripetal xylem present in the leaf-traces at the margin of the pith 
and throughout their outward course into the leaves. 
Leaf-bases massive, crowded. 
Leaves polydesmic, of the type of Cordaites and Mesoxylon . 
All the specimens may be referred to a single species which I have 
pleasure in naming Mesoxylopsis Arberae , sp. nov., after Dr. Agnes Arber, 
F.L.S., who has so kindly aided in the determination of seeds associated 
with the shoots described in the present paper. 1 The characters of the 
species are those of the genus. In the specimens observed the pith is 
relatively small, not exceeding 6 mm. in diameter. 
Conclusion. 
In the first part of the paper the structure of the fertile shoots of 
Mesoxylon midtirame is described. They are found to be identical with the 
axillary shoots, which have been known since the first discovery of the 
species. 
1 See below, p. 18. 
C 
