SPORE WALL ULTRASTRUCTURE IN FOUR SPECIES OF THE LIVERWORT RICCIA

Mature spores of Riccia californica, R. campbelliana, R. sorocarpa, and R. trichocarpa (Marchantiales, Ricciaceae) were examined with both scanning and transmission electron microscopes. The thick-walled spores have species-specific surface sculpturing and similar internal structure. Major surface features include ridges and depressions in areolate or irregular configurations. Finer surface features such as tubercles or papillae may ornament the ridges or depressions. Internally, the sporoderm consists of a thin intine and a three-layered, lamellate exine. The innermost exine lamellae are thin and closely spaced, giving this layer a fibrillar appearance. Lamellae of the middle region are thick in section and have a relatively thick, electron-dense coating. The outermost lamellae are of intermediate thickness and also have a relatively thick electron-dense coating. Lamellae of all regions frequently split, branch and anastomose. The outermost lamella forms the surface features of the spore. Interior lamellae follow the contour of the outermost lamella. Pores usually occur in these four species at the junctures of the triradiate ridge arms and equatorial rim. Pores do not penetrate the entire exine, and they represent localized areas where the outermost group of lamellae have not been formed.