Studies on the development of the air pores and air chambers ofMarchantia paleacea

Summary The preprophase-prophase initial aperture (IA) cells ofMarchantia paleacea undergo a particular sequence of protoplasmic changes, which reflects the establishment of an unusual premitotic polarization. The marking feature of preprophase-prophase thallus cells is the shape of the nucleus which becomes spindle-shaped. This phenomenon accompanies the organization of an extranuclear microtubule (MT) sheath, nucleated and/or organized by distinct polar MT organizing centres (MTOCs).The interphase MTs disappear after activation of polar MTOCs. In preprophase IA cells incomplete preprophase MT bands (PMBs) are organized. They consist of PMB portions which traverse only small portions of the cell cortex at the level of the future cytokinesis and do not form a complete ring. In the same cells other MT bundles, independent of the incomplete PMBs terminate in the cortical cytoplasm abutting on the lower part of the intercellular spaces (ISs) or the surface cavities (SCs). Almost complete or complete PMBs are organized in IA cells in which the plane of PMB formation coincides with that passing through ISs of the same growth.The observations suggest that in preprophase-prophase IA cells ofMarchantia paleacea cortical MTOCs function in regions distant from each other: One region is the PMB cortical cytoplasm, probably that covering the wall edges, and the other is the one adjacent to the lower part of the wall facing the IS(s) or that underlying the SCs. The competition between the cortical MTOCs as well as between them and the polar ones may be responsible for the organization of incomplete PMBs.