Ferns

The gametophyte phase of the fern life cycle begins with a spore. A fern spore is a haploid reproductive cell, which unlike the seeds of higher plants, does not contain an embryo. Fern spores are often dispersed by the wind. Upon germination, a spore gives rise to a green, thread-like tissue, called a protonema. The protonema develops into a prothallus, a small, green, multicellular tissue that is rarely seen in nature. The prothallus has numerous subterranean rhizoids to anchor it to the substrate and absorb nutrients.

Light and other environmental factors control the development of fern gametophytes. In many species, gametophytes kept in darkness do not develop beyond the thread-like protonemal stage. However, illumination with blue or ultraviolet radiation causes the protonema to develop into a heart-shaped prothallus. This is an example of photomorphogenesis, the control of development by light.

Male and female reproductive structures form on the prothallus, and these are referred to as antheridia and archegonia, respectively. Each antheridium produces many flagellated sperm cells which swim toward the archegonia. The sperm cells of some ferns have up to Uluhe ferns (Dicranopteris emarginata) in Hawaii. JLM Visuals. Reproduced with permission.
several hundred flagella each. Each archegonium produces a single egg which is fertilized by a sperm cell.