There are hundreds of exospore units that extend across the gap and into the mesospore in middle stages in megaspores of Selaginella pulcherrima Liebm.ex Fourn and S. haematodes (Kunze) Spring. The mesospore of these megaspores is resistant to acetolysis during middle stages suggesting the presence of sporopollenin. There are also spheroidal structures that are resistant to acetolysis on the surfaces of the outer and inner exospore in both species in middle and mature stages. In late stages the

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... mesospore no longer exists and the inner exospore is a narrow stripe, i.e., there is no longer a space (gap) between outer and inner parts of the exospore. Our interpretation for the loss of the mesospore is that sporopollenin does not become polymerized under physiological conditions to its exceptional state of resistance to reduction, but does so when exposed to native or experimental oxidation; in our case, exposure to the atmosphere by cutting open the megaspore and plunging it into the acetolysis mixture.