Maturation of the megaspore inMarsilea vestita

Publisher Website
Google Scholar
Abstract
The course of megasporogenesis was followed from the simultaneous cleavage of the mother cell at the termination of meiosis to the formation of the compound wall around the single surviving megaspore in the maturing sporangium. The favoured tetrad that yielded the surviving megaspore was always that closest to the stalk of the sporangium. Although no asymmetry could be detected in the tetrad, three of the spores regularly degenerated. The beginning of degeneration was sometimes evident before the release of the spores, but only after the spores had acquired distinct walls. Cytochemical tests indicated that separation of the spores was facilitated by the mother cell wall becoming fluid. Growth of the maturing megaspore was accompanied by conspicuous nucleocytoplasmic interaction. Tubular extensions of the nucleus could be followed in the cytoplasm for more than 1 $\mu $m, but no connections could be detected with any organelles. The central space into which the megaspore grew was probably generated partly by the expansion of the sporangium as a whole, and partly by the digestion of the plasmodial tapetum. The wall of the mature spore was clearly compound, the unstructured exine of the growing spore eventually meeting and fusing with a chambered component formed at the periphery of the tapetum.