Prothalli

A prothallus is the gametophyte generation of certain plants, specifically ferns, horsetails, and some clubmosses. It is a small, often heart-shaped or flattened, thalloid structure that grows independently from the sporophyte generation. The prothallus is typically green and photosynthetic, meaning it can produce its own food. It is anchored to the substrate by rhizoids, which are simple root-like filaments.

The primary function of the prothallus is sexual reproduction. It bears the reproductive organs: the antheridia, which produce sperm, and the archegonia, which produce eggs. Fertilization occurs when sperm from an antheridium swim to an egg within an archegonium on the same or a different prothallus. This fusion of gametes results in the formation of a zygote, which then develops into a new sporophyte plant.

The sporophyte generation, which is the familiar fern frond or horsetail stem, produces spores through meiosis. These spores, upon germination, give rise to the prothallus. Therefore, the life cycle of these plants involves an alternation of generations between a diploid sporophyte and a haploid gametophyte (the prothallus). The prothallus is typically short-lived, often lasting only for a single reproductive season, and its presence is crucial for the continuation of the species.