MEGASPORE WALL IN LYCOPHYTA ULTRASTRUCTURE AND FUNCTION

New contributions on megaspore walls in the genus Selaginella are presented. The study was carried out with electron microscopy (SEM and TEM), light microscopy including phase and differential interference contrast, fluorescence and confocal microscopy. The observations were based on mature material from herbarium specimens and on fresh material in different stages of development. The results refer to: (1) Units: They are coiled rods. Two types have been recognized, one with a central axis and the other without an axis. (2) Patterns of ultrastructure: Two patterns have been distinguished, ordered and slack, according to the type of unit with which they are built. (3) Gaps: They are located within the wall between units, in places where some features change and in the middle and inner parts of the exospore. (4) Exospore ultrastructure in young stages: Two layers have been recognized. Both of them are built by coiled rods. (5) Connections between the tapetum and the plasma membrane of the growing megaspore: During megaspore development there are highly organized structures called wicks. They run across the exospore between units. (6) Mineral deposits within the wall: Deposits of sulphur and potassium in addition to silicon have been detected by X-ray microanalysis.