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Antheridia are structures found in some organisms that produce male sex cells, also known as sperm or gametes. Ferns, mosses, and other seedless plants use them for reproduction, and these structures can also be found in algae and fungi. A microscope and some knowledge of anatomy is necessary to visualize these structures, because they are quite small; many microscopy images of them in various species can be found on biology websites. The female counterpart is the archegonium, with archegonia producing female sex cells.
The structure of the antheridium includes a thin wall of sterile cells that surround haploid cells, which reproduce to make sperm. When the wall is ruptured, the gametes are released, and they can fertilize female sex cells from the same organism or from other organisms to create zygotes with a complete set of diploid DNA.
Organisms use a variety of ways to get the sex cells they produce in their antheridia out into the world. In some cases, the cells are actually motile, and they move through water or a film to reach egg cells and fertilize them. In other cases, they are dispersed by the wind, and sometimes the plant lends a helping hand by exploding the structure to force them out into the air. In many mosses, they can be found at the tips of small stalks that hang over the rest of the plant.
These structures can be seen in organisms that go through a process known as the alternation of generations, in which the organism goes through several distinct phases. Antheridia appear in the gametophyte phase, in which the entire organism is haploid and makes gametes through mitosis. When the resulting gametes fuse, they make a diploid zygote that develops into a diploid spore-producing organism. The spores, in turn, develop into a haploid gametophyte stage, which starts the cycle all over again.
In higher plants such as angiosperms, the antheridia has changed significantly from the form it takes in lower plants. Instead of having a structure to produce male gametes, the plant has single pollen grains that contain single cells, which split to make sperm. In these plants, the gametes fuse to make seeds which contain nutrition to support the developing zygote so that it has a better chance of maturing. By contrast, plants that use antheridia for reproduction rely on favorable conditions for zygote development, although the spores can also last a long time in dormancy while they wait for their opportunity.