Microspore wall organisation and ultrastructure in two species of Selaginella (Lycophyta) producing permanent tetrads

Abstract

Spore morphology and ultrastructural features of the permanent microspore tetrads of Selaginella convoluta and S. lepidophylla were studied with light microscopy, scanning and transmission electron microscopy. The four members of each tetrad in S. convoluta are linked together through a common envelope and form an unbroken complex system of alveoli that connects the equatorial and distal regions of the microspores. In the proximal exospore of microspores, no differentiation referable to an apertural fold was observed but an interruption of the exospore is evident in that area in germinating microspores. The distal exospore is composed of three strata: inner zone, intermediate zone and outer alveolar. In S. lepidophylla tetrads, the four microspores are assembled within a common tetrad envelope and are equatorially connected by a prominent ridge. The distal regions of the microspores are free. In the proximal exospore of microspores, no differentiation referable to an apertural fold was observed. The tetrad envelope appears as a wide common coat, which, on the surface and depending on the regions, is granular or alveolate, centrally perforated and, in section, consists of a single layer or forms a two-layered, more or less intricate, network. This envelope has the same contrast as the exospore outer layer and shows, in its median zone, cavities with opaque contents, quite similar to those of the microspore exospore layer. These two different types of tetrads emphasise the outstanding structural diversity of the microspore sporoderm in living Selaginellaceae.