MICROTUBULE AND ACTIN FILAMENT ORGANIZATION DURING STOMATAL MORPHOGENESIS IN THE FERN ASPLENIUM-NIDUS .1. GUARD-CELL MOTHER CELL

The newly-formed guard cell mother cells (GMCs) of Asplenium nidus are small, lens-shaped and are formed by one or two asymmetrical division s. Their growth axis is parallel to the plane of their future division , a process during which the internal periclinal wall (IPW) is detache d from the partner wall of the underlying cell(s). This oriented GMC e xpansion occurs transversely to a microfibril bundle, which is deposit ed externally to a U-like microtubule (Mt) bundle and a co-localized a ctin filament (Af) bundle. They line the IPW and the major part of the anticlinal walls. The deposition of the microfibril bundle is followe d by the slight constriction of the internal part of the GMCs and the broadening of the substomatal cavity. The IPW forms a distinct bulging distal to the neighbouring leaf margin, as well as a less defined pro ximal one. During the IPW bulging, the Mts and Afs under the external periclinal wall (EPW) attain a radial organization. This is followed b y thinning of the central EPW region, which becomes impregnated with a callose-like glucan. The rest of the EPW becomes unequally thickened. The disintegration of the U-like Mt bundle is succeeded by the organi zation of radial Mt and Af arrays under the IPW. The radial Mt systems , controlling the alignment of the newly-deposited microfibrils, allow the GMC to assume a round paradermal profile. The GMCs form a preprop hase Mt band (PPB) perpendicular to the interphase U-like Mt bundle. T he anticlinal PPB portions appear first and those lining the periclina l walls later. The cytoplasm adjacent to the latter walls retain the r adial Mt systems during early preprophase, simultaneously with the ant iclinal PPB portions. The observations suggest that the GMCs of the fe rn A. nidus obtain a unique form, as a result of a particular polarity established in the cortical cytoplasm of the periclinal walls, in whi ch Mts and Afs appear involved. This polarity persists in cell divisio n and is ''inherited'' to guard cells (GCs). It provides primary morph ogenetic information not only to GMCs but also to GCs.