Maslen . — The Structure of Mesoxylon Sutcliffii (Scott). 401 
of the phloem have much paler contents and are more like the majority of 
the cells in the pericycle and the outer pith as seen in transverse sections. 
They are usually somewhat larger than the cells with darker contents, 
although smaller than the cells of the pericycle (see PI. XXXIII, Fig. 7,/"., 
and PI. XXXV, Fig. 17, b). These, too, appear in longitudinal sections as 
long tubes without obvious cross-walls, although they (or their contents) 
are broken across at irregular intervals. 
Between the cells with contents, occur numerous long, narrow, thin- 
walled empty-looking cells (see PI. XXXV, Fig. 17, c), which may be of the 
nature of sieve tubes, although no actual sieve plates or sieve areas can 
be seen. Other thin-walled empty elements appear to have cross-walls at 
fairly close intervals, and these may perhaps be regarded as phloem 
parenchyma. 
The medullary rays in the phloem are shown on PL XXXV, Fig. 17, 
m.r.j in transverse section. They are not well shown in any of our radial 
sections of the phloem, but the cells appear to be narrow vertically and 
somewhat elongated radially. 
At the outer limit of the phloem, or between that tissue and the peri- 
cycle, there sometimes occurs a more or less well-defined tissue consisting of 
thin-walled cells and cells with very dark contents. This tissue is shown 
on PI. XXXIII, Fig. 7, /"'.,and PI. XXXV, Fig. 1 y,d. It occurs more 
especially opposite to a leaf-trace which is preparing to come out through 
the zone of secondary wood, and when the two bundles of a trace have 
escaped from the xylem and are passing outwards and upwards through the 
pericycle and cortex into the leaf, they are accompanied by two arcs of 
this tissue. This is clearly seen, on PI. XXXIV, Fig. 8, which shows a 
pair of leaf-trace bundles passing out through the pericycle and carrying 
with them the dark cells, c. From their position at the outer limit of the 
secondary phloem, and localization opposite to the primary xylem groups 
of the leaf-trace bundles, it seems probable that this tissue really represents 
the primary phloem groups of the original bundles. 
The phloem, as a whole, appears to be much like that of Cordaites , 
which consists, according to Renault, of sieve tubes and phloem paren- 
chyma with ‘ gum 5 tubes, and, in some forms, bast-fibres. 1 In Poroxylon 
the structure of the phloem is not the same in different species. In 
P. Edwardsii the secondary phloem is extraordinarily well preserved, and 
is made up of distinct alternate tangential bands of sieve tubes and of paren- 
chyma, with broad medullary rays consisting of large cells. Nothing of this 
kind is found in Mesoxylon Sutcliffii. Indeed, the phloem is of quite a 
distinct type, as was at once recognized by Professor Bertrand (one of the 
authors of the original detailed description of Poroxylon ), to whom a section 
of Mesoxylon Sutcliffii was submitted by Dr. D. H. Scott for examination. 
1 Renault, Bassin houiller et permien d’Autun, &c., p. 335. 
