Morphology and ultrastructure of megaspores and microspores of Isoetes savatieri Franchet (Lycophyta)

Abstract

The general morphology, organization, structure and ornamentation of the sporoderm of megaspores and microspores of Isoetes savatieri Franchet have been studied with a stereoscopic microscope, light microscopy, transmission and scanning electron microscopy. The megaspores are trilete, 420–580 μm in equatorial diameter, subtriangular to globose in polar view with an evident equatorial flange (zone). The ornamentation is rugulate. A siliceous cover with a three-dimensional network overlies an exospore sporopolleninous apparently laminar with an equatorial–distal separation of laminae. Each exospore lamina is fused to those of other planes delimiting lacunae. The lacunae located in the outer part of the exospore are filled with silica. The endospore is thick, its structure is fibrillar. In section it is apparently composed of two zones. The microspores are monolete, 35–40 μm long and 20–25 μm wide and elliptic in polar view. A supra-laesural expansion is present. The sculpture is microechinate. A contrasted perispore composed of thin joined threads that form a lacunose structure can be distinguished in the microspore sporoderm. Two parts of the underlying exospore structurally different are evident: a laminar outer part and an inner part, mostly compact. An equatorial–distal separation between both parts of the exospore is evident. The endospore is fibrillar. The ultrastructural similarity of the spores of I. savatieri with those previously studied in Isoetes can be regarded as indicative of the homogeneity of this group of living lycopsids.