104 Salmon. — On the t Genus Fissidens. 
on the stem with a prominent vertical insertion (Fig. 64), 
or it may fail to reach the stem ; (3) the vaginant laminae 
may be equal at the apex, or unequal, with the smaller 
lamina either (in a few cases) ending in the nerve (Fig. 4), 
or, as is usually the case, between the nerve and the margin 
of the leaf ; (4) the nerve may cease below the apex, or be 
strongly excurrent — in very rare cases it is quite rudimentary, 
if not absent. 
Three principal theories have been advanced to explain 
the morphology of the leaf of Fissidens. The first theory — 
the one accepted by bryologists before 1819 — considered 
that the whole structure corresponds to the leaf of other 
Mosses, and that the sheathing part results from a split 
in the thickness of the leaf. De la Pylaie ( 1 ), who mono- 
graphed the genus in 1814, gives this theory in the following 
words: — the leaves present c une fente ou dedoublement dans 
leur epaisseur, qui descend jusqu’a la nervure. Ce dedouble- 
ment . . . resulte de l’ecartement et de la separation de leurs 
deux surfaces, qui forment alors deux lames distinctes.’ 
The second theory was advanced by Robert Brown ( 2 ) in 
1819, in the following words: — £ In Fissidens . . . the leaves 
are universally described as presenting their margin instead 
of their disk to the stem, and as having a doubling of the 
lower half of their inner or upper margin, extending as far 
as the nerve. ... It seems to me a much simpler explanation 
of the apparent anomaly to consider the supposed doubling 
or division of the leaf as its true disk, and the deviation 
from the usual structure as consequently consisting in the 
greater compression of the leaf, and in the addition of 
a dorsal and terminal wing. In support of this view, it may 
be observed that in the lower leaves of the stem both the 
additional wings are greatly reduced in size, and in some 
cases are entirely wanting, as they universally are in the peri- 
gonial leaves which have likewise the more ordinary form, 
being moderately concave and not even navicular.’ 
The view here expressed, that the sheathing part alone 
is the true leaf, and that all the rest is an outgrowth from 
