1 1 8 Salmon. — On the Genus Fissidens. 
not able to find any signs of a 3-rowed arrangement of even 
the young leaves. This points to a very rapid displacement 
of the segments as they are cut off. 
The stems of Bryoxiphium and Fissidens show a very 
similar structure (Figs. 57 and 58). There is a central 
strand of small, very thin-walled cells, surrounded by larger 
cells, the outer layers of which in the old stem become 
strongly cuticularized. 
The fact that the apical cell is 3-sided in Bryoxiphium 
is not unfavourable to the view that this genus may represent 
the ancestral form of Fissidens , as the latter genus clearly 
shows signs of an origin from a Moss with this form of 
apical cell. We see this in the fact, discovered by Hofmeister 
( 20 ), that the underground shoots of Fissidens still grow by 
means of a 3-sided apical cell, and also, that the branches 
of the stems above ground, possess at first a similarly shaped 
apical cell (Leitgeb, 10). 
Leitgeb (loc. cit.) states as his opinion that it is ‘ im 
hochsten Grade wahrscheinlich, dass die Vorfahren unserer 
Fissidenten mit dreiseitiger Scheitelzelle wuchsen und eine 
dreizeilige Blattstellung zeigten, dass also die zweizeilige 
Stellung und ebenso die abnorme Ausbildung der Blatter 
eine erst spater erworbene Eigenschaft sei.’ But with the 
example of Bryoxiphium before us, we must conclude that 
the ancestors of Fissidens , although still possessing a 3-sided 
apical cell, had already acquired the distichous arrangement 
of the leaves. 
Summary. 
Considering the amount of evidence in favour of Robert 
Brown’s theory, and the absence of any valid objections, 
I think this may be safely accepted as satisfactorily explaining 
the morphology of the leaf of Fissidens ; and, further, that 
this ‘leaf’ must be considered as composed of two distinct 
parts, viz. (1) the vaginant laminae, the true leaf ; (2) the 
superior and inferior laminae, formed of one laminar out- 
