Serial Development of Foliar Gemmae in Tortula(Pottiales, Musci), an Ultrastructural Study

Abstract

The development and liberation mechanism of foliar gemmae have been studied by electron microscopy in two mosses, Tortula latifoliaBruch and Tortula papillosaWils. The gemmae develop on the adaxial surface of mature leaves from single initial cells on both the lamina and costa in T. latifoliabut only on the costa in T. papillosa. Elongation of the initial cell is associated with the deposition of a highly extensible new wall whilst the old wall and cuticle in the apical dome rupture. The first division is transverse and separates a short basal cell embedded in the foliar tissue and a distal cell, or gemma primordium, protruding from the leaf surface. Subsequent divisions of the gemma primordium give rise to a six-to-eight-celled globose gemma with mucilaginous outer walls. During gemma development the basal cell produces a new wall and elongates again whilst the common wall with the gemma splits apart centripetally along the boundary between the old and new wall in the basal cell; plasmodesmal connections are gradually severed and eventually the young gemma remains connected to the basal cell only by mucilage. After separation of the first-formed gemma, the basal cell may expand and produce a second gemma by the same mechanism. The whole process may be repeated several times resulting in the formation of a chain of gemmae stuck together by mucilage and which are liberated only when the leaves are fully hydrated. Accumulation of abundant lipid deposits in the gemmae after symplasmic isolation reflects considerable photosynthetic autonomy.