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Flowering plants contain microsporangia in the anthers of stamens (typically four microsporangia per anther) and megasporangia inside ovules inside ovaries. In all seed plants, spores are produced by meiosis and develop into gametophytes while still inside the sporangium. The microspores become microgametophytes (pollen).
Spores form part of the life cycles of many plants, algae, fungi and protozoa. [2] They were thought to have appeared as early as the mid-late Ordovician period as an adaptation of early land plants. [3] Bacterial spores are not part of a sexual cycle, but are resistant structures used for survival under unfavourable conditions. [4]
Under certain stressors such as heat or starvation, plants select for microspore embryogenesis. It was found that over 250 different species of angiosperms responded this way. [ 6 ] In the anther , after a microspore undergoes microsporogenesis , it can deviate towards embryogenesis and become star-like microspores.
Sori occur on the sporophyte generation, the sporangia within producing haploid meiospores. As the sporangia mature, the indusium shrivels so that spore release is unimpeded. The sporangia then burst and release the spores.
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.
The sporophyte produces spores (hence the name) by meiosis, a process also known as "reduction division" that reduces the number of chromosomes in each spore mother cell by half. The resulting meiospores develop into a gametophyte. Both the spores and the resulting gametophyte are haploid, meaning they only have one set of chromosomes.
The main dispersive spores are asexual, self-motile spores called zoospores, which are capable of chemotaxis (movement toward or away from a chemical signal, such as those released by potential food sources) in surface water (including precipitation on plant surfaces). A few oomycetes produce aerial asexual spores that are distributed by wind.
Plant spores are most obvious in the reproduction of ferns and mosses. However, they also exist in flowering plants where they develop hidden inside the flower. For example, the pollen grains of flowering plants develop out of microspores produced in the anthers. Reproductive spores grow into multicellular haploid individuals or sporelings.