ULTRASTRUCTURE OF SPOROGENESIS IN A MOSS,DITRICHUM PALLIDUM. III. SPORE WALL FORMATION

Ultrastructural evidence indicates that marked cytoplasmic polarity occurs during wall and aperture ontogeny in spores of the moss (Musci), Ditrchum pallidum (Hedw.) Hampe. Shortly after cytokinesis, an extensive system of microtubules underlies the entire distal spore surface where exine deposition is initiated. These microtubules appear to be focused on the plastid. The apposition of slips nearly of membrane dimension contributes to the forming exine. As the lamellate exine thickens and extends to the proximal surface, the plastid and associated nucleus migrate to the proximal surface where an elaborate system of microtubules involved in aperture development is generated. The exine gradually loses its stratiform character, becoming homogenous and eventually papillate. At maturity, the spore wall consists of four layers, the outermost perine, the exine, a separating layer, and the intine. The aperture is a complex, localized modification of these layers on the proximal surface. It consists of a pore containing a fibrillar material surrounded by a thin annulus.