MICROTUBULE ORGANIZATION, MESOPHYLL CELL MORPHOGENESIS, AND INTERCELLULAR SPACE FORMATION IN ADIANTUM-CAPILLUS VENERIS LEAFLETS

Mesophyll cells (MCs) of Adiantum capillus veneris are elongated and highly asymmetric, bearing several lateral branches and forming a meshwork resembling aerenchyma. Young MCs are polyhedral and display oppositely arranged walls and transverse cortical microtubules (Mts). Their morphogenesis is accomplished in three stages. At first they become cylindrical. Intercellular space (IS) canals, containing PAS-positive material, open through their junctions and expand laterally. During the second stage the cortical Mts form a reticulum of bundles, externally of which an identical reticulum of wall thickenings, containing bundles of parallel cellulose microfibrils, emerges. MCs do not grow in girth in the regions of wall thickenings, where constrictions form and new ISs open. Thus, MCs obtain a multi-lobed form. At the third morphogenetic stage MCs display a multi-axial growth. During this process, additional Mt rings are assembled at the base of cell lobes accompanied by similarly organized wall thickenings-cellulose microfibrils. Consequently, cell lobes elongate to form lateral branches, where MCs attach one another, while the IS labyrinth broadens considerably. Colchicine treatment, destroying Mts, inhibits MC morphogenesis and the concomitant IS expansion, but does not affect IS canal formation. These observations show that: (a) MC morphogenesis in A. capillus venerisis an impressive phenomenon accurately controlled by highly organized cortical Mt systems. (b) The disposition of Mt bundles between neighbouring MCs is highly coordinated. (c) The perinuclear cytoplasm does not appear to be involved in cortical Mt formation. Cortical sites seem to participate in Mt bundling. (d) Although extensive IS canals open before Mt bundling, the Mt-dependent MC morphogenesis contributes in IS formation.