Development and liberation of cauline gemmae in the moss Aulacomnium androgynum (Hedw.) Schwaegr. (Bryales): an ultrastructural study

The leafy shoots of the moss Aulacomnium androgynum form clusters of gemmae borne terminally on long pseudopodial axes. The gemmae arise from single initial cells produced by the activity of a superficial meristem. Mature gemmae comprise an apical and a basal cell with four to seven cells forming two, sometimes three, tiers in between. The basal cell is connected to the tip of the pseudopodium by a uniseriate filament consisting of an abscission (tmema) cell and a stalk cell. The first division of the initial cell produces a proximal cell and a distal cell. The proximal cell elongates without further division forming the stalk of the gemma; the distal cell gives rise to a lower and an upper cell by transverse division. The upper cell divides repeatedly by oblique septa forming the apical and middle cells of the gemma; the basal cell and tmema cell arise from a transverse division of the lower cell. The first two divisions in gemma development are highly asymmetrical and exogenous, i.e. preceeded by cell expansion. A broad interphase cortical band of microtubules is associated with intercalary cellular growth during this stage. Subsequent gemma development follows an endogenous pattern with cellular expansion following the completion of proliferative divisions and involving a conventional system of cortical microtubules. While elongating to about four times its original length without deposition of a distinct new wall the tmema cell undergoes cytoplasmic degeneration and eventually breaks, causing gemma liberation. The stalk cell elongates about eight-fold and its contents also degenerate after gemma liberation. Plasmodesmata in the basal and stalk cells are obliterated by the deposition of additional wall materials. The highly electron-opaque outer walls of the mature gemmae and tips of the stalk cells are water-repellent. The gemmae are dispersed either in water films or by air currents.