2008 Volume 59 Issue 3 Pages 239-247
Mizutania riccardioides Furuki & Z. Iwatsuki is unique in having an irregularly-branched, unistratose plant body (thallus). Although the resemblance between thalli of Mizutania and some fern gametophytes has been noted, the mode of development of the irregularly branching thalli of Mizutania has not yet been clarified. We examined the developmental morphology of thallus branching, focusing on the behavior of the apical meristem, which usually has a single apical cell that functions as an initial cell. When branched, the branch apical cells arise in the young merophytes derived from the original apical cell. This mode of branch apical cell formation is not unique and is found in other liverworts, but in M. riccardioides some branch apical cells stop dividing and disappear through periclinal divisions soon after initiation, while others give rise normally to branch thalli. Which branch apical cells are lost and which are retained as initial cells appears to be arbitrary. Furthermore, the apical cell in the growing branch thallus sometimes abruptly ceases to function and forms small branch thalli with fully differentiated non-meristematic cells. In conclusion, the arbitrary loss of apical cell activity and differences in duration and/or onset of the timing of branch apical cell activity results in highly irregularly shaped thalli.
