Megaspore development in Selaginella The gap and the mesospore

A gap in the exospore and the presence of a mesospore is found to be a normal part of development in the ten species of Selaginella we have studied. The early spore wall consists of an exospore and a mesospore forming successively on the plasma membrane of the megaspore protoplast when it is 10-15 mu m in diameter. Enlargement of the exospore and mesospore creates a central space, the lumen of the megaspore, around the megaspore protoplast. After that there is a vast enlargement of the exospore and a relatively small enlargement of the mesospore. The exospore splits close to its contact with the mesospore forming a gap over equatorial and distal regions. The gap becomes greatly expanded and becomes filled with lipids, PAS-positive carbohydrates, proteins and is crossed by wicks. Experiments with solutions of different osmolality on fresh megaspores show that the exospore and mesospore are not osmotic barriers. The mesospore appears not to be resistant to acetolysis at the many stages tested but exospore is resistant. Thus the mesospore size and shape is retained by the inner exospore that enveloped the mesospore. At maturation the mesospore undergoes lysis and absorption. At the beginning of germination stages an endospore forms at the inner part of the exospore. This inner part of the exospore, that adhered to and enveloped the mesospore, becomes pressed near to the bulk of the exospore. Until pregermination stages the megaspore protoplast is small (10-20 mu m in diameter).