MICROTUBULE ORGANIZATION AND CELL MORPHOGENESIS IN 2 SEMI-LOBED CELL-TYPES OF ADIANTUM-CAPILLUS-VENERIS L LEAFLETS

Protodermal cells of Adiantum capillus-veneris leaflets are polyhedral , displaying regularly arranged cortical microtubules transverse to th e main cell axis. The nascent epidermal cells partly detach from the u nderlying mesophyll cells by formation of intercellular spaces, contai ning PAS-positive material. In early differentiating upper epidermal c ells discrete U-like bundles of cortical microtubules form on the inte rnal periclinal and the anticlinal walls. In contrast, microtubules ar e randomly scattered along the external periclinal wall. Microtubule b undles of neighbouring anticlinal walls of epidermal cells exhibit an alternating disposition but are directly opposite to those of underlyi ng mesophyll cells. Epidermal cell wall is locally reinforced by thick enings arising under the microtubule bundles and including parallel ce llulose microfibrils. The pattern of wall thickenings reflects that of the microtubule bundles. The internal periclinal epidermal cell regio n expands at the sectors free of wall thickenings, forming several lob es. Simultaneously, intercellular spaces open at the thickened regions of anticlinal walls, which finally become wavy. In contrast, the exte rnal periclinal wall does not form any lobes but remains smooth. As a result, epidermal cells become 'semi-lobed'. The lobes of lower epider mal cells are less prominent. Mesophyll cells surrounding the endoderm is are also 'semi-lobed'. Their morphogenesis is achieved by the same mechanism. Colchicine treatment inhibits the 'semi-lobed' morphogenesi s of epidermal cells and mesophyll cells surrounding the endodermis an d the concomitant intercellular space opening. These observations reve al that the primary event of 'semi-lobed' cell morphogenesis is the or ganization of two different patterns of the cortical microtubule cytos keleton in the same cell.