1 2 2 Notes. 
systematic position of Isoetes , some of which I will now endeavour 
to meet by anticipation. 
In the first place, it may be objected that, in the growth in 
thickness of its stem, Isoetes differs from the Filicinae and indicates a 
relationship with the extinct Lycopodinae. In reply to this it may be 
pointed out that secondary growth in thickness of the stem is by 
no means a peculiarly Lycopodinous character; and further, that 
some indication of such secondary thickening is to be found in 
the rhizome of existing Ophioglosseae. 
Secondly, the absence of a single apical cell in the growing-point 
of either stem or root may be urged as an objection to the in- 
corporation of Isoetes with the Filicinae. This is not, however, 
an objection of any weight; for among the Lycopodinae, on the one 
hand, the presence of a single apical cell is common in Selaginella, 
and among the Filicinae, on the other, the presence of a single apical 
cell is not universal, there being in the roots of the Marattiaceae 
a group of apical cells. 
It may be added here that Russow 1 has already drawn at- 
tention to certain histological resemblances between Isoetes and the 
Ophioglosseae. 
Further, the resemblance between the male and female gameto- 
phytes of Isoetes and Selaginella may be urged as a ground for 
keeping these genera together, and therefore also for retaining Isoetes 
among the Lycopodinae. It has been already pointed out that the 
two genera do differ in this respect, and it may be further suggested 
that the reduction of the gametophytes of Isoetes , as compared with 
those of the Hydropterideae, is just what might be expected in higher 
and lower groups of the same series. In fact, the comparison of the 
gametophytes of Isoetes and Selaginella rather supports the view that 
they are forms, not belonging to one group, but occupying cor- 
responding positions in two different series : that is to say, that 
Isoetes occupies in the Filicinae the same relative position as Sela- 
ginella in the Lycopodinae. 
The presence of a ligule in both Isoetes and Selaginella might also 
be brought forward as a reason for classing them together; but, 
when contrasted with the wide difference in the class-characters, 
the importance of this common feature is but small. Moreover, it 
1 Russow, Vergleichende Untersuchungen, Mem. de l’Acad. imp. de St. 
Petersbourg, ser. 7, t. xix, 1872, p. 192. 
