Salmon . — On the Genus Fissidens. 1 1 7 
a few cells, variable in number, at the keel of the leaf, 
which are thickened in such a way as to leave a more or 
less rounded lumen ; frequently the nerve-cells shade off 
gradually into those of the laminae. The gradual appearance 
of the outgrowth is shown in Figs. 49-52. A wide, but 
weak and ill-defined, nerve appears in the outgrowth ; this 
is shown in section in Fig. 52, taken along the line a , b, in 
Fig. 46. The perichaetial leaves (Fig. 47) are nerveless, but 
have still a prominent appendage. Bryoxiphium and Sorapilla 
are, therefore, related to Fissidens in the structure of their 
leaves ; the former also shows other signs of relationship. 
The stem of Fissidens , as is well known, possesses the pecu- 
liarity of growing by means of a 2-sided apical cell. From this 
cell, segments are cut off alternately to the right and left, so 
that the leaves from the first have a distichous arrangement 
(Fig. 53). It is often stated that in this respect Fissidens 
stands alone amongst Mosses. Lorch ( 18 ) has, however, stated, 
although without giving any details, that a 2-sided apical cell 
is found in species of Phyllogonium , and Goebel ( 21 ) has 
recently said that the same is present f in Fissidens , Phyllo- 
gonium, and perhaps still other Mosses with distichously 
arranged leaves. 1 In all other Mosses the stem has a 3-sided 
apical cell, from the three sides of which segments are cut off 
regularly (e.g. as in Mnium , Fig. 54), and the later distichous 
arrangement of the leaves, where it exists, is due to subsequent 
displacement. 
Unfortunately from the herbarium material at my disposal, 
I have been unable to study in detail the apical growth of 
Bryoxiphium and Sorapilla. Campbell ( 19 ), however, has 
investigated the former, and states that the apical cell is 
3-sided, and that therefore the distichous arrangement of 
the leaves is secondary. The arrangement of the young leaves 
in the bud is remarkably similar, however, in Bryoxiphimn 
and Fissidens (cf. Figs. 55 and 56) ; in both cases the hairs, so 
prominent in the bud, persist in the axils of the mature leaves. 
The distichous arrangement of the leaves in Bryoxiphium 
is maintained up to the very apex of the stem, and I was 
