The Sporangia of Stauropteris oldhamia. 115 
there is no reason to doubt that the whole structure as known to 
us at present, is nothing but the highly ramified petiole and rachis 
of a compound frond. The stem on which it was borne is still to 
be discovered. 
The large petioles, 3 mm. or more in diameter, show, in trans¬ 
verse section, the well-known cruciate form of the xylem, described 
and figured by Williamson, the character on which Binney’s generic 
name was based. The general form of the stele, as a whole, is 
approximately square, the phloem filling up the bays between the 
xylem-arms, and sometimes extending to the middle, so as to more 
or less completely break up the wood. 
Though 4-armed the wood is not properly described as tetrarch, 
for there are often at least two groups of small elements, no doubt 
the protoxylem, at the periphery of each arm. The tracheides are 
scalariform ; Williamson was unable to detect any true spirals, but 
they occur, though sparingly, at the protoxylem-angles. 
The phloem contains large sieve-tubes, often well-preserved, 
and showing the sieve-plates on their lateral walls very plainly. 
They are of the usual Fern-type. 
Leaving the pericycle and endodermis for consideration on 
another occasion, we may pass on to the cortex. This consists of 
long cells, often trumpet-shaped at the ends; the outer layers are, 
as a rule, narrower and more sclerotic than the inner. In well- 
preserved specimens the periphery of the rachis is occupied by a 
zone of delicate, lacunar tissue, the cells of which are often radially 
elongated, like a very lax palisade. This zone is bounded by the 
epidermis, in which scattered stomata are found. The whole no 
doubt constituted the assimilating tissue of the rachis, which, as 
we shall see, was very necessary in the absence of vegetative 
leaflets. The assimilating tissue extends onto branches of all 
orders, and may even be found on the fine ultimate ramifications 
which bear the sporangia (Fig. 2c). The rachis is abundantly and 
repeatedly branched, and as Williamson showed, the branches are 
given off in pairs, both branches of the pair springing from the 
same side of the parent-axis. 1 The branches are of various orders ; 
those of intermediate dimensions commonly have a three-armed, 
instead of a four-armed xylem, and the two structures are often 
found in connection, as figured by Williamson. 2 The ultimate 
ramifications are excessively delicate, from 150//. to 200// in diameter. 3 
1 Williamson, loc. cit., p. 687, Figs. 25a and b, 26. 
“ Loc. cit., Fig. 26. 
» Williamson, loc. cit., p. 686, Fig. 24. 
