Salmon . — On the Genus Fissidens. 119 
growth from the back of the leaf, in which a nerve has 
been developed. 
We can even, perhaps, get a glimpse at the evolution of 
the ‘ leaf.’ Looking at the often twisted and contorted pro- 
longations of the upper leaves of Bryoxiphium (a genus of 
three rare species), we may say that the attempt to produce 
a vertical expansion from the leaf was here frustrated by 
following the plan of elongating the whole leaf on one side, 
so that the prolongation remained double on that side to 
the apex ; in Sorapilla (with one species known only from 
a single station in the Andes) a step in the right direction 
was taken in producing only a vertical outgrowth from the 
back of the leaf, but the nerve here is very poorly developed, 
and unable, perhaps, to keep in position any considerable 
vertical outgrowth ; finally, we see that Fissidens , by enor- 
mously enlarging the outgrowth, and dwarfing and vertically 
compressing the true leaf, has made the whole assume the 
symmetrical form of a normal leaf, furnished with a strong 
nerve, and has attained the reward of spreading over the 
tropical and temperate regions of the world with its 500 
species. 
Part II. Classification. 
The systematic position of Fissidens has been treated very 
differently by different authorities. The abnormal structure 
of the leaf has usually led to its being placed in a separate 
Order Fissidentaceae. Having regard only to the dicranoid 
peristome, many authors place this Order close to Dicranaceae. 
Others, on account of some of the species being cladocarpous, 
consider that the Order is intermediate between the Acro- 
and Pleurocarpi. 
Schimper places the Order between Leucobryaceae and 
Seligeriaceae. Muller includes Fissidenteae in the first section 
Distichophylla of the Stegocarpous Acrocarpi, with the tribes 
Schistostegeae, Drepanophylleae, and Distichiaceae. Mitten 
places Fissidens , Bryoxiphium , Sorapilla , and Eustichia in 
