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a spore from which develops the female prothallium (in gymnosperms, the primary endosperm nucleus; in angiosperms, the embryo sac). The megaspore arises as a result of the division of the megasporocyte (mother cell). In Pteridophyta the number of mature megaspores in the megasporangium varies. The Isoetes have many megaspores. The Lepidodendron and Sigillaria have eight to 16; the Selaginella, two to four; the water ferns, only one. Megaspores grow into female prothallia either inside (Lepidodendron, Selaginella, Salvinia) or outside (Marsilea, some Selaginella) the megasporangium. The first stages in the development of the prothallium occur inside the megaspore; the membranes of the megaspore then open and the parts of the prothallium that bear the archegonia emerge (in Salvinia they turn green). In seed plants, four megaspores usually form in each nucellus (the central part of the ovule).

References in periodicals archive ?
Studies have also reported the correlation between the ploidy level and the size of spores of either the megaspore or microspore (Luebke and Budke, 2003), or even for both mega- and microspores (Small and Hickey, 2001).
Leaf bases pale to brown; leaves usually less than 1 mm wide at middle; megaspores papillate, more than 450 um in diameter (0.
The remaining polyploids are larger still, ranging to up to decaploids (10X) with 700-800 [micro]m diameter megaspores.
SEM photomicrographs of megaspores were imaged at 200X and microspores were imaged at 2000X.
Microspores and megaspores are strikingly different in size and shape.
2009), in the southern part of South America there is a group of species with reticulate megaspores that is very complex taxonomically.
Isolated megaspore tetrads of Stauropteris burntislandica.
Sporophyte production was rapid, resulting in 100% and 99% emergence from megaspores from plants A and B, respectively, within the fourth week.
Development of the ovule, megaspores and megagametophyte of Aletris aurea and their connection with the systematics of the genus.
Masses of megaspores and microspores can be collected from pots of S.
The subgenus Euazolla, characterized by three megaspore floats and septate glochidia, included four species: A.
macrotuberculata Marsden based on the ornamentation of the megaspores (i.