a New Fern from the Coal-Measures. 2 1 5 
association of concentric and collateral vascular strands in 
fern-fronds, there is a considerable difference between the 
distinctly collateral bundles of Myeloxylon and the more 
typical fern-like bundles of Rachiopteris. As regards the 
presence or absence of xylem -parenchyma, it is a fact on 
which very great stress must not be laid. In the petioles 
of Marattiaceae we occasionally find a few parenchymatous 
elements associated with the xylem-tracheids, but as a rule 
they are absent. De Bary 1 , in speaking of concentric bundles, 
refers to those in which the xylem is entirely composed of 
tracheids, and those in which rows of parenchymatous cells 
are intermixed with them : he says — ‘ The two conditions 
are distributed according to species and perhaps genera, 
not according to the form of the bundle/ Potonie 2 3 , in a 
useful paper Ueber die Z us am mensetzang dev Leitbiindel 
bei den Gefasskryptogamen, makes use of Troschel’s term 
‘ amylom ’ for xylem-parenchyma, and, among other ex- 
amples, he cites the occurrence of such tissue in the bundles 
of Marattia laxa 3 ; he shows that amylom-cells may be 
present or absent in different parts of the vascular system of 
the same plant. Troschel’s term is applied not merely to 
those parenchymatous cells which are intermixed with the 
tracheids, but includes those immediately surrounding the 
tracheid-group ; in speaking of the occurrence of such elements 
in the fossil petioles and the recent Marattiaceae it is only 
the amylom-cells intermixed with the tracheids that are 
referred to. The marked contrast between Myeloxylon and 
Rachiopteris Williamsoni as regards the xylem-parenchyma 
should probably be kept in view as a distinction of some 
importance in questions of affinity. On comparing the 
sections of R. Williamsoni with those of recent fern-petioles, 
we find a much closer correspondence with such a Fern as 
Angiopteris than with any of the Leptosporangiate genera. It 
is true that there may be a natural bias in favour of the 
1 Comp Anat., p. 344. 
2 Jahrb. k. bot. Gart. und bot. Mus. Berlin, II, 1883. 
3 Potonie, loc. cit. p, 244 : PI. VIII, Fig. 4. 
