Brown Spots Underneath a Fern Leaf

Spores

• The raised bumps on the undersides of a fern frond are not the actual reproductive spores. The bumps are called sori -- the singular is sorus -- and they are where spores are produced and stored until they are mature. When the spores mature, the sori burst open and release millions of spores into the wind or water.

Prothallus

• The conditions that initiate spore germination vary from species to species, but generally moisture is an important factor. Once the right conditions are met, a spore will germinate and grow into a structure called a prothallus. The prothallus is heart-shaped, and at its base, the pointed end of the heart, are root-like filaments called rhizoids. Among the rhizoids are antheridia, the male, sperm-producing sex organs. Above them, at the rounded part of the heart, are the archegonia, the female, egg-producing sex organs.

Fertilization

• Once mature, the antheridia can release sperm. Water must be present for the sperm to be able to swim for an egg-baring archegonia. Sperm can fertilize an egg from the archegonia on the prothallus it came from, but it is best for the survival of the fern species for the sperm to reach a different prothallus than its own.

Young Ferns

• Once fertilization is complete, the egg matures into what is called a zygote, which is essentially the beginning of a new fern. As the first, tiny fronds appear, the prothallus shrivels. In its infancy, a fern can look rather moss-like and is very sensitive to heat and drought. When fronds get big enough, they'll uncurl in the typical fiddlehead fashion, unrolling as they emerge from the ground.