STUDIES ON THE SPOROGENOUS LINEAGE IN THE MOSS TIMMIELLA-BARBULOIDES .8. SPORODERM DEVELOPMENT

The formation of the sporoderm in the moss Timmiella barbuloides (Brid.) Moenk. (Pottiales) was investigated by electron and fluorescence microscopy. Exine is the first layer formed. It is initiated during late cytokinesis on the distal surface of the tetrad members by centripetal apposition of lamellae just outside the plasma membrane. The exine then extends along the margins of the spore up to the proximal face where it contributes to the formation of the aperture. Exine deposition is closely related to two extensive systems of microtubules that develop sequentially from discrete microtubule organizing centres associated with the plasma membrane at distal and proximal poles of spores. After spore liberation from the tetrads the exine becomes homogeneous. Discontinuous perine ornamentations of extrasporal origin are then deposited onto the outer surface of the exine while the aperture development is completed. The intine is the last and innermost layer formed. Its deposition is restricted to the proximal hemisphere of the spore with thickness decreasing from the pole to equator.