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 g emmae borne terminally on long pseudopodial axes. The gemmae arise fro m single initial cells produced by the activity of a superficial meris tem. Mature gemmae comprise an apical and a basal cell with four to se ven cells forming two, sometimes three, tiers in between. The basal ce ll is connected to the tip of the pseudopodium by a uniseriate filamen t consisting of an abscission (tmema) cell and a stalk cell. The first division of the initial cell produces a proximal cell and a distal ce ll. The proximal cell elongates without further division forming the s talk of the gemma; the distal cell gives rise to a lower and an upper cell by transverse division. The upper cell divides repeatedly by obli que 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 c ortical band of microtubules is associated with intercalary cellular g rowth during this stage. Subsequent gemma development follows an endog enous pattern with cellular expansion following the completion of prol iferative divisions and involving a conventional system of cortical mi crotubules. While elongating to about four times its original length w ithout deposition of a distinct new wall the tmema cell undergoes cyto plasmic degeneration and eventually breaks, causing gemma liberation. The stalk cell elongates about eight-fold and its contents also degene rate after gemma liberation. Plasmodesmata in the basal and stalk cell s are obliterated by the deposition of additional wall materials. The highly electron-opaque outer walls of the mature gemmae and tips of th e stalk cells are water-repellant. The gemmae are dispersed either in water films or by air currents. (C) 1996 Annals of Botany Company