47 
Zygopteris Grayi of Williamson . 
Another characteristic feature of the undivided trace is the presence of 
a median island or strand of internal xylem, accompanied by parenchyma 
(PL I, Phot 8 ; PL III, Figs, i and 2, xA). The island has an irregularly 
elongated shape, and one or more smaller islets may be detached from it 
(Phot. 8). These, however, die out as the trace is followed upwards, and 
the median strand becomes more regular. This strand is destined to form 
the internal xylem of the axillary stele (PL I, Phot, i, a.s . ; PL IV, Fig. 9). 
I have found it continuous at all levels, both in the Shore specimen and in 
the best one of Williamson’s (PL IV, Fig. 12 ; PL V, Fig. 13). The 
doubt expressed by Dr. Kidston (TO, pp. 452, 454) as to the presence 
of a ‘ pith ’ at the place where the axillary stele separates from the leaf- 
trace is evidently to be explained by defects of preservation, as he himself 
suspected. It must, however, be emphasized that in no case is a true pith 
present in Zygopteris Grayi , either in the main stele, the undivided leaf- 
trace, or the axillary stele. In all cases the central tissue contains numerous 
tracheides, as proved by longitudinal as well as transverse sections. 
The whole form and structure of the undivided trace in the Shore 
specimen show that this organ is essentially a leaf -trace rather than the 
stele of a branch. It constitutes, as already stated, the common base of the 
foliar bundle and the axillary stele ; in the Shore specimen the foliar 
bundle characters predominate, while in the Williamson specimens, at 
least those which have been figured, the stele characters are more obvious, 
so that the whole has been regarded as a branch (compare PL III, Fig. 1, 
with Pl. IV, Fig. 12). Such an idea would scarcely have arisen if specimens 
like that from Shore had first come under observation. The question will 
be more fully discussed later in the paper (p. 57). 
Emission of the Leaf-trace. 
The whole process can be followed in the Shore specimen. 
If we start with that arm of the stele which has last given off a leaf- 
trace (e. g. arm 5, shown in PL I, Phots. 4 and 5) we find that it is short, with 
a rounded outline, and without any sign of the peripheral loops. The 
corresponding internal ray penetrates the arm for half its length or more. 
Otherwise there is little or no differentiation. At a slightly higher level 
the internal ray extends further outwards, and the tracheides external 
to its termination become markedly smaller than the rest. The extension 
of the ray is at first one-sided (see Phot. 1, arm 1 ; Phot. 2, arms 1 and 2 ; 
Phot. 6, arm 5). This is the first indication of a peripheral loop. Further 
up, a second branch of the internal ray appears, so that it becomes forked. 
The branches very nearly reach the surface, from which they are only 
separated by bands of small tracheides (Phots. 3 and 4, arms 1 and 2). 
The structure at this level is of much interest, as we here see continuity 
