Ultrastructural Changes Associated with Spore Formation in Sporangia and Sporangiola of Thamnidium elegans Link

Abstract

Fully formed pre-cleavage sporangia and sporangiola of Thamnidium elegans Link were bounded by a primary wall plus a thick, internal secondary wall layer. In sporangia in late pre-cleavage, Golgi-like cisternae were associated with groups of cytoplasmic vesicles of characteristic size and appearance which were not found in sporangia containing large cleavage vesicles. In both sporangia and sporangiola, protoplast cleavage was effected by enlargement of endogenous cleavage vesicles each containing a lining layer of variable appearance, mutual fusion of cleavage vesicle membranes and fusion of cleavage vesicle membranes with the plasmalemma. Golgi-like cisternae and small vesicular profiles were present in sporangium protoplasts at all stages of cleavage vesicle enlargement. In sporangia, the columella zone was delimited by cleavage vesicles and separated from the sporogenous zone by a fibrillar wall. A similar wall, which sometimes protruded to form a small columella, was formed in sporangiola. Recently delimited spore protoplasts were bounded by plasmalemma membrane derived from cleavage vesicle bounding membrane and sporangium or sporangiolum plasmalemma and surrounded by an investing layer derived from cleavage vesicle lining material. The investing layer at first appeared single, but later two electron opaque profiles were discernible. The spore wall was formed between the investing layer and the plasmalemma. Walls of sporangia and sporangiola which contained fully formed spores consisted of the primary layers only.