Ferns have 2 stages to their life cycle. The obvious adult fern and another small, sexual, “secret” stage that most people don’t ever see. The adult fern is called a sporophyte because they produce tiny spores that are spread by the wind in great numbers. Each spore is a group of cells that contain the same genetic makeup and chromosomes as the adult fern.
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When spores land on a damp surface in the bush they grow into a thin, heart–shaped green leaf-like structure about the size of a small finger nail. This is called a prothallus. It is the gametophyte stage of the fern life cycle because it produces gametes - sperm and eggs - as animals do. The sperm and eggs have half the number of chromosomes as the adult sporopyhyte.
Prothalli growing on fine sand in our spore growing trays.
Prothallus upside down showing the fine rhizoids (like roots)
When it is wet underneath the prothallus, sperm swim from special glands amongst the rhizoids to another prothallus nearby where they fertilize an egg in special female glands under the rhizoids. The fertilized egg now has the original number of chromosomes – half from each of the “mother” and “father” prothalli. It then grows into the adult, sporophyte stage of the fern that we all recognise.
2 new fronds growing from the fertilised eggs under their prothalli. Each one continues to grow along with a new rhizome and roots to form the adult fern.
It can take 2 or more years to produce a new fern in a pot this way in the nursery.
Auckland University produced an excellent 3D animated video showing the stages in the Fern Life Cycle. Many of the depicted events are microscopic.