172 W. T. GORDON 
is none in that of the stem or of the lower parts of the petiole. The inner cortex has 
generally all decayed, but small patches sometimes appear in the islands formed between 
the pinna-trace-bar and the petiole-trace. It has always the appearance of a delicate 
parenchyma. 
Numerous groups of sporangia, probably synangia, occur scattered through the 
blocks containing the petioles and stems of M. duplex. There are generally four 
sporangia in each group, but there is no evidence that these were borne by the pinnules 
of this plant 
HIsToLoGy OF THE STEM. 
The stem which has just been referred to M. duplex is of peculiar interest both on 
account of its simple structure and because it is of considerable phylogenetic import- 
ance. In transverse section it is circular in outline (Pl. I. fig. 1, and Pl. IV. fig. 45), 
and about 1°8 mm. in diameter. Two zones may easily be distinguished, the inner 
consisting of a mixture of parenchyma and tracheidal tissue, and the outer being made 
up entirely of tracheides. Pl. IV. fig. 45 probably gives the best idea of the distribu- 
tion of the tracheides and parenchyma in transverse section. (Although a petiole-trace 
connected to the stem has been cut away at the bottom of the figure, it has not disturbed 
the more central tissues.) In other examples the tracheides are almost absent, and in 
Pl. IV. fig. 47 such a specimen is figured; only one tracheide (¢) is present, and it is 
immersed in the centre of a thin-walled parenchyma (p). The general circular shape of 
the stem is distorted in this specimen, and a petiole-trace is shown connected with the 
stem. ‘This trace apparently belonged to a petiole which had been torn away, leaving 
a ragged stump still connected to the stem, and the tissues of the trace are so crushed 
that one of the protoxylem groups is unrecognisable, and the whole trace very irregular 
in outline. 
In most specimens, however, the parenchyma has been ruptured and only fragments 
of the delicate cell-walls remain. As a result the tissue is seen better in transverse 
than in longitudinal section. In one example (Pl. IV. fig. 46) a peculiar condition was 
discovered. ‘There is no parenchyma present in the stem, and there is a distinct radial 
arrangement of the tracheides at a. This stem is very much smaller than any of the 
others, and I have only seen it in one preparation. } 
I am inclined to think that it is an unequal dichotomy of the stem, and it will be 
referred to later. In the meantime the absence of conjunctive parenchyma is worthy 
of notice. 
The tracheides of this inner zone are generally small in diameter and vary consider- 
ably. The largest are only between 45 and 50, while the small ones are 30m and 
under. Those nearest the periphery are generally the largest, and they abut on the 
inner tracheides of the outer xylem zone, which are not much larger than the outer 
tracheides of the mixed pith. The thickenings on the walls of the inner elements are 
scalariform in the smallest but reticulate in the largest. In the same tracheide the’ 
