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Modern heterosporous plants such as many ferns exhibit endospory, in which a megagametophyte is fertilized by a microgametophyte all while still inside the spore wall, gaining nutrients from the inside of spore. [6] Both heterospory and endospory seem to be one of the many precursors to seed plants and the ovary.
While heterospory is the norm among all plants with seeds, such as the flowering plants and conifers, it is very rare among other groups of plants. Also, most heterosporous plants produce their two kinds of sporangia in different places on the plant.
Salviniales are all aquatic and differ from all other ferns in being heterosporous, meaning that they produce two different types of spore (megaspores and microspores) that develop into two different types of gametophyte (female and male gametophytes, respectively), and in that their gametophytes are endosporic, meaning that they never grow outside the spore wall and cannot become larger than ...
Megaspores, also called macrospores, are a type of spore that is present in heterosporous plants. These plants have two spore types, megaspores and microspores. Generally speaking, the megaspore, or large spore, germinates into a female gametophyte, which produces egg cells.
Heterosporous plants, such as seed plants, spikemosses, quillworts, and ferns of the order Salviniales produce spores of two different sizes: the larger spore (megaspore) in effect functioning as a "female" spore and the smaller (microspore) functioning as a "male".
In heterosporous plants, sporophylls (whether they are microphylls or megaphylls) bear either megasporangia and thus are called megasporophylls, or microsporangia and are called microsporophylls. The overlap of the prefixes and roots makes these terms a particularly confusing subset of botanical nomenclature.
Salviniaceae (/ s æ l ˌ v ɪ n i ˈ eɪ s i ˌ iː /), [1] [2] is a family of heterosporous ferns in the order Salviniales. [3] The Salviniaceae contain the two genera Azolla and Salvinia, [4] with about 20 known species in total. [5] The oldest records of the family date to the Late Cretaceous. [6]
All flowering plants are heterosporous, that is, every individual plant produces two types of spores. Microspores are produced by meiosis inside anthers and megaspores are produced inside ovules that are within an ovary. Anthers typically consist of four microsporangia and an ovule is an integumented megasporangium.