Microsporangia

Microsporangia are sporangia that produce microspores, the male spores of heterosporous plants. They occur in seed plants as the pollen sacs within the anthers of angiosperms and in pollen cones or pollen sacs of gymnosperms, and in some ferns and lycophytes as part of reproductive sporophylls. Each microsporangium contains numerous diploid microsporocytes derived from sporogenous tissue.

During microsporogenesis, each microsporocyte undergoes meiosis to yield four haploid microspores. In many species the microspores remain briefly bound as tetrads before separating. Microgametogenesis follows: each microspore undergoes mitosis to form a male gametophyte, typically a two-cell pollen grain consisting of a vegetative (tube) cell and a generative cell; in many angiosperms the generative cell later divides to produce two sperm cells. In gymnosperms, pollen grains often deliver a germinating pollen tube to the ovule, and the sperm cells are delivered via the tube. In contrast, in many ferns and related lineages, microsporangia on sporophylls also produce microspores that become part of the male gametophyte in their respective life cycles.

The microsporangial wall consists of multiple layers, commonly including an outer epidermis, an endothecium, one or more middle layers, and an innermost tapetum. The tapetum nourishes developing microspores and contributes materials to the pollen wall. Dehiscence of the microsporangium releases pollen grains, enabling pollination and subsequent fertilization. Microsporogenesis and microgametogenesis are crucial steps that connect the diploid sporophyte to the haploid male gametophyte in seed plants.