514 LIFE-HISTORIES 



more upon moist earth or rocks; in these the thallus shows 

 the broad liver-Hke lohing especiallj' well, and often appears 

 as a flat rosette (Fig. 338, A, B). The more aquatic forms 

 have narrow, much-branched, ribbon-like lobes (P, Q, R), 

 and bear a striking resemblance to such algae as carrageen, 

 while the forms with disk-like thallus (./, K), are closely 

 similar to forms of sheath-algse. In both erj'stalworts and 

 sheath-alga? 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 conseciuence of this continuous terminal growth and branch- 

 ing is that wlien the okler 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 

 tlie thallus dividing like a terminal cell and so producing a tiny 

 bud or brood-body which, separating, Ijecomes 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 Riocia grow older the,v 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 Coleocha'te but soon become immersed in the thallus through 

 the growth of surrounding cells. Although strictly homologous 

 with the gametangia of Coleochajte those of the liverwort are some- 

 what more elaborate in structure. The male gametangium (Fig. 330, 

 A-D) includes a number of cells producing motile gametes each Imy- 

 ing two flagella like the male gametes of Coleochsete and differing 

 from them chiefly in having a more slender body. The female 

 gametangium (E, a") is a 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 

 l)onie on the same thallus, tluit is to say, the thallus is biscxunl: while 

 in other cases, a thallus has but one kind of gametangimn, making 

 it thus vjiiscxual. 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 rutlimentary sporophyte or embryo. The imier 

 ' Ar-chc-go'ni-urn < Gr. archv, first ; gono'^, gcneratiou. 



