– Definition of Microsporangium:
– Microsporangia produce microspores for male gametophytes in various vascular plants.
– Found in gymnosperms, angiosperms, seed plants, spike mosses, and certain aquatic ferns.
– Microsporangia produce microsporocytes, which generate four microspores through meiosis.
– Microsporocytes in gymnosperms and angiosperms become pollen grains after division.
– Heterosporous plants evolved microsporangia and megasporangia independently.
– Evolution of Microsporangium:
– Heterosporous plants developed microspores in microsporangia during the Devonian period.
– Fossils show endosporic gametophytes in these plants.
– Gametophytes developed within spores, similar to modern heterosporic plants.
– Evolution of separate microsporangia and megasporangia occurred in various plant groups.
– This evolutionary development dates back to the Devonian period.
– Structure of Microsporangium:
– Anthers in angiosperms consist of actively dividing meristematic cells.
– Anther lobes develop pollen sacs through differentiation of archesporial cells.
– Different layers form the pollen sac wall, including epidermis, endothecium, and tapetum.
– Anther development leads to the formation of pollen sacs at the corners.
– Tapetal cells may be uni-, bi-, or multinucleate with dense cytoplasm.
– Microsporangium in Angiosperms:
– Anthers in angiosperms go through a developmental process to form pollen sacs.
– The differentiation of archesporial cells leads to the formation of pollen sacs.
– Anther lobes develop into four pollen sacs at the corners.
– The wall layers of pollen sacs consist of various cell types.
– Development of pollen sacs starts with the differentiation of archesporial cells.
– References:
– Bateman and Dimichele’s study on heterospory as a key innovation in plant evolution.
– Stewart and Rothwell’s book on paleobotany and plant evolution.
– These references provide insights into the evolutionary history of plants.
– The study of microsporangium contributes to understanding plant reproduction.
– References highlight the importance of microsporangia in the plant kingdom.
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Microsporangia are sporangia that produce microspores that give rise to male gametophytes when they germinate. Microsporangia occur in all vascular plants that have heterosporic life cycles, such as seed plants, spike mosses and the aquatic fern genus not species Azolla. In gymnosperms and angiosperm anthers, the microsporangia produce microsporocytes, the microspore mother cells, which then produce four microspores through the process of meiosis. Microsporocytes are produced in the microsporangia of gymnosperm cones and the anthers of angiosperms. They are diploid microspore mother-cells, which then produce four haploid microspores through the process of meiosis. These become pollen grains, within which the microspores divide twice by mitosis to produce a very simple gametophyte.
Heterosporous plants that produced microspores in microsporangia and megaspores in separate megasporangia evolved independently in several plant groups during the Devonian period. Fossils of these plants show that they produced endosporic gametophytes, meaning that their gametophytes were not free-living as in bryophytes but developed within the spores, as in modern heterosporic vascular plants.
In angiosperms, a very young anther (the part of the stamen that contains the pollen) consists of actively dividing meristematic cells surrounded by a layer of epidermis. It then becomes two-lobed. Each anther lobe develops two pollen sacs. Then, a two-lobed anther develops four pollen sacs that situate at four corners of the anther. Development of pollen sacs begins with the differentiation of archesporial cells in the hypodermal region below epidermis at four corners of the young anther. The archesporial cells divide by periclinal division to give a subepidermal primary parietal layer and a primary sporogenous layer. The cells of the primary parietal layer divide by successive periclinal and anticlinal divisions to form concentric layers of pollen sac wall.[citation needed]
The wall layers from periphery to center consist of:
- A single layer of epidermis between, which becomes stretched and shrivels off at maturity
- A single layer of endothecium. The cells of endothecium have fibrous thickenings.
- One to three middle layers. Cells of these layers generally disintegrate in the mature anther
- A single layer of tapetum. The tapetal cells may be uni-, bi- or multinucleate and possess dense cytoplasm. The cells of the primary sporogenous layer divide further and give rise to diploid sporogenous tissue.