514 LIFE-HISTORIES 
more upon moist earth or rocks; in these the thallus shows 
the broad liver-like lobing especially well, and often appears 
as a flat rosette (Fig. 338, 4, B). The more aquatic forms 
have narrow, much-branched, ribbon-like lobes (P, Q, R), 
and bear a striking resemblance to such alge as carrageen, 
while the forms with disk-like thallus (J, K), are closely 
similar to forms of sheath-algze. In both crystalworts and 
sheath-alge a lobe elongates by the continued division of a 
single terminal cell, which by its occasional forking gives rise 
to similar branches. Compare Fig. 314 with Fig. 338, P. 
One consequence of this continuous terminal growth and branch- 
ing is that when the older parts die and decay the newer parts are 
distinct plants which have thus arisen by a sort of vegetative re- 
production. No swarm-spores are produced, but the thallus often 
propagates non-sexually by single mature cells in various parts of 
the thallus dividing like a terminal cell and so producing a tiny 
~ bud or brood-body which, separating, becomes a distinct plant. The 
main structural difference between the alga and the liverwort- 
thallus is a somewhat more advanced differentiation of the latter. 
~ As the cells of Riccia grow older they may give rise on the lower 
surface to filamentous pseudo-roots and sometimes scale-like or 
tongue-like pseudo-leaves, while at the upper surface they may 
form a firm protective layer. Gametangia arise on the upper surface 
as in Coleochete but soon become immersed in the thallus through 
the growth of surrounding cells. Although strictly homologous 
with the gametangia of Coleochzte those of the liverwort are some- 
what more elaborate in structure. The male gametangium (Fig. 339, 
A-D) includes a number of cells producing motile gametes each hav- 
ing two flagella like the male gametes of Coleochete and differing 
from them chiefly in having a more slender body. The female 
gametangium (H, a”) isa flask-shaped multicellular organ containing 
a single female gamete. A female gametangium thus constructed 
is distinguished as an archegonium,! the female gamete being called 
an egg-cell. In some cases both male and female gametangia are 
borne on the same thallus, that is to say, the thallus is bisexual; while 
in other cases, a thallus has but one kind of gametangium, making 
it thus uniserual. In the bisexual plants close-fertilization can 
doubtless occur; while in the unisexual, only cross-fertilization is 
possible. Fertilization is effected by a single male gamete, which 
because of its slender form is able to make its way down the pro- 
jecting neck of the archegonium to the egg-cell. The zygote be- 
comes surrounded by a cellulose wall, and through repeated division 
forms a spherical mass of cells which at first are all much alike. 
This mass is a rudimentary sporophyte or embryo. The inner 
1 Ar-che-go/ni-um < Gr. arche, first; gonos, generation. 
