Development and Liberation of Cauline Gemmae in the Moss Aulacomnium androgynum (Hedw.) Schwaegr. (Bryales): An Ultrastructural Study

The leafy shoots of the mossAulacomnium androgynum form clustersof gemmae borne terminally on long pseudopodial axes. The gemmaearise from single initial cells produced by the activity ofa superficial meristem. Mature gemmae comprise an apical anda basal cell with four to seven cells forming two, sometimesthree, tiers in between. The basal cell is connected to thetip of the pseudopodium by a uniseriate filament consistingof an abscission (tmema) cell and a stalk cell. The first divisionof the initial cell produces a proximal cell and a distal cell.The proximal cell elongates without further division formingthe stalk of the gemma; the distal cell gives rise to a lowerand an upper cell by transverse division. The upper cell dividesrepeatedly by oblique septa forming the apical and middle cellsof the gemma; the basal cell and tmema cell arise from a transversedivision of the lower cell. The first two divisions in gemmadevelopment are highly asymmetrical and exogenous, i.e. preceededby cell expansion. A broad interphase cortical band of microtubulesis associated with intercalary cellular growth during this stage.Subsequent gemma development follows an endogenous pattern withcellular expansion following the completion of proliferativedivisions and involving a conventional system of cortical microtubules.While elongating to about four times its original length withoutdeposition of a distinct new wall the tmema cell undergoes cytoplasmicdegeneration and eventually breaks, causing gemma liberation.The stalk cell elongates about eight-fold and its contents alsodegenerate after gemma liberation. Plasmodesmata in the basaland stalk cells are obliterated by the deposition of additionalwall materials. The highly electron-opaque outer walls of themature gemmae and tips of the stalk cells are water-repellant.The gemmae are dispersed either in water films or by air currents. Abscission; asexual reproduction; bryophytes; morphogenesis; microtubules; ultrastructure