— 6o- 
and in certain species of Scapania and Sphenolobus, the lobes are approxi- 
mately equal in size, and the leaves ought not to be described as either incu- 
bous or succubous. In the majority of cases, however, where the complicate 
condition occurs, the lobes are unequal in size, the dorsal lobe being the 
larger in certain species and the ventral in others. In the Lejeuneae ( Fig . 
13), in Frullania (Fig. 14 ), Radula and Porella (Fig. 13), the dorsal lobe is 
the larger and the leaves are described as incubous : in many species of Scap- 
ania and in Diplophylleia (Fig. 16) the ventral lobe is the larger and the 
leaves are described as succubous. These conditions may also be best seen 
by diagrams (Figs. 17- iq). In the genus Ptilidium the leaves are normally 
quadrifid and at the same time complicate, the keel occurring between the two 
middle divisions. In Pt. ciliare , which is one of. the commonest and most 
conspicuous species in the eastern United States, the leaves are beautifully 
fringed on the margin; and this condition is carried to an extreme in the still 
more beautiful Trichocolea tomentella , where the leaves present the appear- 
ance of being divided almost to the base into an innumerable number of 
delicate hairs. 
The leaves in many of the Hepaticae, through their arrangement, 
overlapping, lobing and folding, doubtless assist the plant materially in 
absorbing and retaining water. This is seen especially well in the two spe- 
cies just described, where the whole plant is practically permeated in all direc- 
tions by capillary spaces, which can take up and hold water like a sponge. 
In certain genera this function is assumed by a definite part of the leaf, 
which becomes hollowed out and is know as the “water-sac.” Among our 
northern genera this structure is best studied in Radula , in the Lejeuneae 
(Fig. 13), and in Frullania (Fig. 14). all of which, as noted above, are 
characterized by complicate-bilobed leaves, the dorsal lobe being the larger* 
In all these forms the water-sac is formed wholly or in part by the ventral 
lobe or. as it is often called, the “lobule,” to distinguish it from the dorsal 
division of the leaf, called simply the “ lobe.” In Radula and in the Lejeu- 
neae, the free edge of the lobule is closely appressed to the lobe except in the 
outer part, and the region of the leaf near the keel becomes inflated and a’cts 
as the sac, the water gaining entrance through the minute opening in the 
outer part where lobe and lobule are not in contact. In these cases both lobe and 
lobule enter into the formation of the water-sac. In Frullania a part of the 
lobule itself becomes hollowed out into a hood-like organ, open at one end 
and blind at the other ; here the entire sac is formed by the lobule. 
In comparison with the leaves the underleaves exhibit much less variety, 
as has already been noted. They are almost invariably transversely 
attached to the stem, the line of attachment being straight or nearly so j 
sometimes, however, they are decurrent, and the line of attachment becomes 
more or less arched. The latter condition is well seen in Porella (Fig. 13), 
where the decurrent base of the underleaf is sometimes longer than the free 
portion. Here again, as in the leaves, the simplest type is undivided, but 
the apex although sometimes broad and rounded as in Archilej eunea clyp- 
eata (Fig. 13 ), and in Porella (Fig. 13). is usually sharply pointed, the 
underleaf itself assuming a lanceolate or subulate form. These pointed 
