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Asporangium (fromLate Latin, from Ancient Greek σπορά (sporá) 'seed' and ἀγγεῖον (angeîon) 'vessel';pl.:sporangia)[1] is an enclosure in whichspores are formed.[2] It can be composed of asingle cell or can bemulticellular. Virtually allplants,fungi, and many other groups form sporangia at some point in theirlife cycle. Sporangia can produce spores bymitosis, but in land plants and many fungi, sporangia produce genetically distincthaploid spores bymeiosis.
In some phyla of fungi, the sporangium plays a role inasexual reproduction, and may play an indirect role insexual reproduction. The sporangium forms on the sporangiophore and containshaploidnuclei andcytoplasm.[3]Spores are formed in the sporangiophore by encasing each haploid nucleus and cytoplasm in a tough outermembrane. During asexual reproduction, these spores are dispersed via wind andgerminate into haploidhyphae.[4]
Although sexual reproduction in fungi varies between phyla, for some fungi the sporangium plays an indirect role in sexual reproduction. ForZygomycota, sexual reproduction occurs when the haploid hyphae from two individuals join to form azygosporangium in response to unfavorable conditions. The haploid nuclei within the zygosporangium then fuse intodiploid nuclei.[5] When conditions improve, the zygosporangium germinates, undergoes meiosis and produces a sporangium, which releases spores.
Inmosses,liverworts andhornworts, an unbranchedsporophyte produces a single sporangium, which may be quite complex morphologically. Most non-vascular plants, as well as manylycophytes and most ferns,[clarification needed] arehomosporous (only one kind of spore is produced). Some lycophytes, such as theSelaginellaceae andIsoetaceae,[7]: 7 the extinctLepidodendrales,[8] and ferns, such as theMarsileaceae andSalviniaceae areheterosporous (two kinds of spores are produced).[7]: 18 These plants produce bothmicrospores andmegaspores, which give rise to gametophytes that are functionally male or female, respectively. Most heterosporous plants there are two kinds of sporangia, termedmicrosporangia and megasporangia.
Sporangia can be terminal (on the tips) or lateral (placed along the side) of stems or associated with leaves. Inferns, sporangia are typically found on theabaxial surface (underside) of the leaf and are densely aggregated into clusters calledsori. Sori may be covered by a structure called an indusium. Some ferns have their sporangia scattered along reduced leaf segments or along (or just in from) the margin of the leaf. Lycophytes, in contrast, bear their sporangia on theadaxial surface (the upper side) of leaves or laterally on stems. Leaves that bear sporangia are calledsporophylls. If the plant is heterosporous, the sporangia-bearing leaves are distinguished as either microsporophylls or megasporophylls. In seed plants, sporangia are typically located withinstrobili or flowers.
Cycads form their microsporangia on microsporophylls which are aggregated into strobili. Megasporangia are formed into ovules, which are borne on megasporophylls, which are aggregated into strobili on separate plants (all cycads are dioecious).Conifers typically bear their microsporangia on microsporophylls aggregated into papery pollen strobili, and the ovules, are located on modified stem axes forming compound ovuliferous cone scales. Flowering plants contain microsporangia in the anthers ofstamens (typically four microsporangia per anther) and megasporangia insideovules 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). The megaspores become megagametophytes (embryo sacs).[citation needed]
Categorized based on developmental sequence,eusporangia andleptosporangia are differentiated in the vascular plants.
A cluster of sporangia that have become fused in development is called asynangium (pl.synangia). This structure is most prominent inPsilotum andMarattiaceae such asChristensenia,Danaea andMarattia.
Acolumella (pl. columellae) is a sterile (non-reproductive) structure that extends into and supports the sporangium of some species. In fungi, the columella, which may be branched or unbranched, may be of fungal or host origin.Secotium species have a simple, unbranched columella, while inGymnoglossum species, the columella is branched. In someGeastrum species, the columella appears as an extension of the stalk into the spore mass (gleba).[9]
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