Mesoxylon Lomaxii and M, poroxyloides. 1021 
diameter, most of them over 30 /x. The rays are almost invariably uniseriate, 
ranging in height from one to about ten cells ; the diameter of a ray is 
commonly from 20 to 25 /x, the cells being about 36 yuc in height. At the 
end towards the pith the rays become broader, and radial divisions of their 
cells are frequent. The wood of M. poroxyloides thus differs from that of 
M. Lomaxii in the somewhat smaller size of the tracheides, with a corre- 
sponding diminution in the number of rows of pits, and, so far as observed, 
in the absence of very high medullary rays. There is pretty close agree- 
ment with the wood of M. Sntcliffii , as described by Mr. Maslen. The 
greater development of the inner spiral and scalariform zone of tracheides 
in the secondary wood is characteristic of M. poroxyloides . 
The Phloem and Pericycle. 
The phloem in the type specimen forms a zone o*8 mm. to 1 mm. in 
breadth (PI. LXXXVIII, Fig. 15). Nearly the whole thickness is made 
up of radially seriated, secondary elements. The phloem is very fairly 
preserved, and is closely similar in structure to that of M. Sntcliffii , of 
which Mr. Maslen has given a full description (Maslen, ’ll, p. 400). The 
elongated elements with dense contents are very conspicuous ; in some the 
contents are black, in others light brown, but the two appear to pass over 
into one another, and I cannot find any sharp distinction between the two 
forms. It may be mentioned that cells with dark contents also occur in the 
phloem rays, which are often dilated towards the exterior. It is probable, 
as suggested by Mr. Maslen in the case of his species, that the long 
elements with no obvious contents are the sieve-tubes. I postpone, however, 
any detailed account of the phloem to a later paper, as I have recently 
received from Mr. Lomax a series of sections from a remarkably well 
preserved stem, closely similar to M. poroxyloides as regards the phloem, 
though differing in some other details. Beyond the phloem comes another 
fairly broad zone, which may best be regarded as the pericycle. This 
contains a number of large sacs with dark contents (Fig. 15, In 
longitudinal section the sacs often appear to be divided up into cells, but 
this may only be due to a breaking up of the contents. 
The Cortex. 
On the outer border of the pericycle is a layer of internal periderm, 
very unequally developed, and in some places reaching a thickness of about 
a dozen cells. Many of the periderm cells are flattened and rather thick- 
walled, suggesting a true cork. There is, however, no sign of any general 
desiccation of the outer tissues, so it is evident that they had not yet been 
completely cut off by cork from communication with the water supply of 
the stem. At many places the periderm is double, an outer layer or arc 
having formed in the adjacent cortical tissue. On general grounds one would 
