SPORANGIUM: CHARACTERISTICS, PARTS AND FUNCTIONS - BIOLOGY - 2024

The sporangium is defined as the capsule or sac-like structure, present in many plants and fungi, within which reproductive spores are formed and stored. The word sporangium comes from two Greek words; "Sporus", which means spore, seed and "angio", which means conduit, vessel or container.

Fungi, plants, and other organisms produce sporangia at some stage in their life cycle. In sporangia, spores can be produced by mitosis-like cell division.

Figure 1. Sporangium with spores inside. Source: UBA Wikimedia Commons.

However, in many species of fungi and in the vast majority of land plants, sporangia are the structures where meiosis generally occurs, producing spores with only one set of chromosomes (haploid).

Sporangia in fungi

Some groups of fungi, considered the most primitive or least evolved, have sporangia or capsules where spores are formed. These sporangia contain haploid cytoplasm and nuclei, and are located at the ends of specialized aerial hyphae, called sporangiophores.

These primitive fungi differ from more evolved fungi in that their asexual spores are endogenous, that is, they form in structures within the fungus. The sporangium plays an important role in asexual reproduction and an indirect role in sexual reproduction.

Each spore forms within the sporangium by surrounding itself with a strong outer membrane, haploid nucleus, and cytoplasm. These spores are dispersed through various mechanisms (depending on the type of fungus) and through asexual reproduction they germinate on suitable substrates, producing haploid hyphae.

Fungi sporangia that form spores with flagella (zoospores)

Primitive aquatic and terrestrial fungi form within their sporangia flagellate spores (zoospores) that allow them to swim.

Zoospores of primitive aquatic fungi can swim in the surrounding water thanks to their flagellum. The zoospores of primitive terrestrial fungi are only released from the sporangium when it rains, that is, when there is a lot of humidity in the environment.

The zoospores of the primitive terrestrial fungi swim using the flagellum as a propellant appendage, among the particles of the soil wet by rain. They can also swim on wet plant surfaces, for example on leaves after rain.

Fungi sporangia that form spores without flagella

Some types of fungi also have sporangia that form spores that do not have flagella, or ability to move, but are dispersed with the wind.

Fungal sporangia and their role in sexual reproduction

The sexual reproduction cycle of fungi also varies depending on the group or phylla to which the fungus belongs. For some fungi, the sporangium is indirectly involved in sexual reproduction.

For example, for the fungi of the Zygomycota group, sexual reproduction occurs when, under unfavorable environmental conditions, the compatible haploid hyphae of two individuals unite, fusing their cytoplasms and forming a zygosporangium.

The haploid nuclei of the zygosporangia also fuse, forming diploid nuclei, that is, with two series of each chromosome. When external environmental conditions improve and are favorable, the zygosporangium can germinate, undergo meiosis-like cell division, and produce a sporangium that ruptures its capsule and releases spores.

Sporangia on land plants

In terrestrial plants such as mosses, liverworts and Anthocerotophytas, a sporophyte (multicellular structure of plants in its diploid phase, which produces haploid spores) unbranched, forms a single sporangium, with a complex structure.

It is pertinent at this point to point out that mosses are small plants of the Briophyta group, not vascular, that is, they do not have conducting vessels.

The liverworts are also Briophytes, not vascular, small perennial herbs, from very humid areas, with a shape similar to that of a liver, which, unlike mosses, have unicellular rhizoids. Anthocerotophyta is a group of very primitive vascular higher plants.

A haploid cell contains a single set of chromosomes in the nucleus. A diploid cell contains two sets of chromosomes in its nucleus.

The vast majority of non-vascular plants (which do not have sap-conducting vessels), such as several of the Licophytas (primitive plants) and most of the ferns, produce only one type of spores (they are homosporic species).

Some liver plants, most Licophytas and some ferns, produce two types of spores and are called heterosporic species. These plants produce two types of spores: microspores and megaspores, which give rise to gametophytes.

Gametophytes originating from microspores are male and gametophytes originating from macrospores are female. In some cases, the two types of spores form in the same sporangium.

In most heterosporic plants there are two types of sporangia, called microsporangia (which produce microspores) and macrosporangia (which form macrospores). The sporangia can be terminal, if they form at the ends, or lateral, if they are located along the sides of the stems or leaves.

Fern sporangia

In ferns, sporangia are commonly located on the underside of leaves and form dense aggregates called sori. Some ferns have the sori in leaf segments or along the edge of the leaves.

Figure 2. Sporangia on the underside of the leaves of a fern, the sori are observed. Source: pixabay.com

Lycophyta sporangia

Lycophytas plants have their sporangia on the upper surface of the leaves or laterally on the stems.

Cicadaceae sporangia

The Cicadaceae family consists of a single genus of plants, the genus Cycas. They are plants similar to palm trees, native to the continents Asia, Africa and Oceania.

Cicadaceae have leaves that form aggregates called strobili. They form their microsporangia on the strobili. Megasporangia are formed within ovules, in strobili of separate dioecious plants, that is, different male and female plants.

Sporangia in conifers

Coniferous plants, such as pines, have their microsporangia in leaf aggregates or pollen in strobili. The ovules are located on modified stem axes.

Sporangia on seed plants

In all plants that have seeds, spores are formed by meiosis-type cell division and develop within the sporangium, becoming gametophytes. The microspores develop into microgametophytes or pollen. The megaspores form the megagametophytes or embryo sacs.

Sporangia on flowering plants

Flowering plants contain microsporangia in the anthers of the stamens and megaesporangia in the ovules, within the ovaries of the flowers.

Parts of the sporangium

The internal structure of the sporangia is constituted by a sterile, non-reproductive structure that extends inside and is called a columella. This fulfills functions of support of the sporangium. In fungi, the columella may or may not be branched.

On the other hand, the capsule or sac that makes up the sporangium has a strong and resistant wall, which breaks, releasing the spores under special conditions depending on the type of fungus.

Functions of the sporangium

The sporangium fulfills important functions of production and protective deposit of spores. It is the place where spores are produced and stored, until favorable external conditions for their release arise.

References

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