An Ultrastructural Study of Microspore Wall Morphogenesis in Selaginella tamariscina (Beauv.) Spring (Selaginellaceae)

Abstract

The morphogenesis of the microspore wall in Selaginella tamariscina was examined using transmission electron microscopy. The sporoderm of S. tamariscina consists of three layers: the endospore, exospore, and perispore. The endospore is the thick, innermost layer, which has a low electron density. The exospore consists of three distinct sub-layers. The innermost sub-layer consists of several sheets of tripartite lamellae (lamellar layer). It is located on the proximal face of the microspore. The middle sub-layer is a uniform, thick, homogeneous layer (inner homogeneous layer). The outermost sub-layer is even thicker, conspicuously sculptured, and also consists of homogeneous material (outer homogeneous layer). The layers of the exospore begin to form just after meiosis in that order. The endospore forms at the same time as the outer homogeneous layer. Finally, the perispore is deposited on the exospore. The perispore is an electron-opaque layer that is tightly attached to the surface of the exospore. The endospore and the exospore lamellar layer appear to be derived from the cytoplasm of the microspore, while the inner and outer homogeneous layers of the exospore and the perispore are derived from the tapetum.