16 CONNECTICUT GEOL. AND NAT. HIST. SURVEY. [Bull. 
important differences in this case being in the gametophytes. 
In most of the Metzgeriacee the gametophyte is a thallus, 
but a few of the genera show a more or less complete differen- 
tiation into stem and leaves. The plants are usually composed 
of parenchyma throughout, but a few thalloid species develop 
a very primitive conducting tissue composed of elongated cells 
with lignified walls. The archegonia are borne on the upper 
surface of the gametophyte or of a special branch, and do not 
directly terminate its growth. In many cases a protective 
structure is developed outside the calyptra, and this sometimes 
assumes the form of a pseudoperianth as in the Marchantiacez. 
The Jungermanniacez are sometimes called Scale Mosses, 
the gametophyte being invariably a leafy stem. Most of the 
species are prostrate, and the plants show a distinct dorsi- 
ventrality, even when ascending or erect. The leaves are 
normally alternate and arranged in three ranks, two of which 
are turned toward the light and the third toward the sub- 
stratum. The leaves of this third rank are called underleaves, 
and are usually much smaller than the others and different 
from them in form. Sometimes they are so~much reduced in 
size that they can scatcely be demonstrated, and in a few 
genera they are absent altogether. The two ranks of large 
leaves usually spread out in such a way that the whole shoot 
acquires a strongly flattened appearance, very characteristic 
of the family as a whole. 
The leaves as a rule exhibit no cell differentiation whatever, 
and are invariably destitute of midribs. They show, however, 
a great deal of variation in form and in the way in which 
they are attached to the stem. They are sometimes undivided, 
sometimes variously toothed, lobed, or deeply cleft; they are 
sometimes developed in one plane, sometimes variously folded; 
they are sometimes attached by a continuous line, sometimes 
by two lines which meet at an angle. In a few genera the 
leaves develop peculiar organs, known as water sacs, in which 
water may be temporarily retained. The branches sometimes 
show a differentiation into those which bear normal leaves 
and those which assume a flagelliform appearance, the leaves 
in the latter case being strongly reduced or even absent 
altogether. The flagelliform branches frequently perform the 
