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 norm al part of development in the ten species of Selaginella we have studied. T he early spore wall consists of an exospore and a mesospore forming success ively on the plasma membrane of the megaspore protoplast when it is 10-15 m u m in diameter. Enlargement of the exospore and mesospore creates a centra l space, the lumen of the megaspore, around the megaspore protoplast. After that there is a vast enlargement of the exospore and a relatively small en largement of the mesospore. The exospore splits close to its contact with t he mesospore forming a gap over equatorial and distal regions. The gap beco mes greatly expanded and becomes filled with lipids, PAS-positive carbohydr ates, proteins and is crossed by wicks. Experiments with solutions of diffe rent osmolality on fresh megaspores show that the exospore and mesospore ar e not osmotic barriers. The mesospore appears not to be resistant to acetol ysis at the many stages tested but exospore is resistant. Thus the mesospor e size and shape is retained by the inner exospore that enveloped the mesos pore. At maturation the mesospore undergoes lysis and absorption. At the be ginning of germination stages an endospore forms at the inner part of the e xospore. This inner part of the exospore, that adhered to and enveloped the mesospore, becomes pressed near to the bulk of the exospore. Until pregerm ination stages the megaspore protoplast is small (10-20 mu m in diameter).