Recognition and characterization of inner exospore wall layers in modern and fossil lycopsids — The mesospore

Abstract

Abstract Differing interpretations of sporoderm development in megaspores of Selaginella have resulted in confusion surrounding use of the term mesospore. Different ontogenetic events result in one of two distinct structural types in the innermost exospore. The first type consists of a thin, highly compressed layer (referred to as an inner separable layer; ISL) which separates, along with the endospore, from the remainder of the overlying exospore. This layer forms if a viable protoplast persists in the megaspore. Growth of the viable protoplast compresses the ISL tightly to the inside of the overlying exospore although a line of separation can still be detected even in fully mature megaspores. The second structural type, which may be termed a “mesospore proper”, develops if the protoplast aborts. In the latter case, there is evidence to suggest that the aborting protoplast adds additional material (if only residues of cellular autolysis) to the inside of the mesospore. This protoplast abortion and deposi...