514 LIFE-HISTORIES 



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

 the broad hver-hke lobing especially 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-algae. In both crystalworts and 

 sheath-algae 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 bj- 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 maj' 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. Gamctangia arise on the upper surface 

 as in Coleocha'tc but soon l^ecome immersed in the thallus through 

 the growth of surrounding cells. Although strictly homologous 

 with the gametangia of C'oleochicte 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 Coleocha'te 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 

 borne on the same thallus, that is to say. the thallus is bi-^^exual; while 

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

 it thus unisexunl. In the bisexual plants close-fertilization can 

 doubtless occur; while in the unisexual, only cross-hn'tilization is 

 possible. Fertilization is effected by a single male gamete, which 

 because of its slender form is ai)le to make its way down the pro- 

 jecting neck of the archegonium to the egg-cell. The zygote be- 

 comes surrounded by a cellulose v.^all, and through repeated division 

 forms a spherical mass of cells which at first are all much alike. 

 This mass is a rudimentary sporoph,vte or embryo. The inner 

 1 Ar-che-go'ni-um < Gr. arche, first; gonos, generation. 



