Ferns are pteridophytes. The life cycle of a fern involves two distinct phases known as the alternations of generations. The first is called the gametophyte stage, and the second is called the sporophyte stage. The sporophyte stage produces spores that develop into gametophytes. Gametophytes then produce female and male sex cells (or gametes). The female gametes are fertilized and develop into a new sporophyte so that the cycle can continue.
Gametophyte Cell Structure
Gametophytes are haploids, meaning that they have a single set of chromosomes. They develop from spores that germinate and develop into the prothallus. When it is mature, small sections of the prothallus separate into sexual organs. These sexual organs include the antheridia (which produce male sex cells) and the archegonia (which produce female sex cells), according to the article "An Introduction to the Structure of Ferns and their Allies" on the Australian National Botanic Gardens website. In most cases, the prothallus is green and photosynthetic and is able to develop on moist rocks as well as on the ground.
The male sex cells move more freely than the female sex cells. They also have cilia (sensory organelles) that allow them to travel to the archegonia, where fertilization will take place. After fertilization, a new sporophyte develops and the gametophyte dies.
The prothallus is thin and a bit heart-shaped. It has a thick central region that creates the antheridia, archegonia and the rhizoids (root-like structures). The prothallus lacks vascular tissue.
Sporophyte Cell Structure
Sporophytes are diploids, meaning that they have two sets of chromosomes. They are large, photosynthetic, green and long-living. According to the article "An Introduction to the Structure of Ferns and their Allies," what people recognize as the fern plant is actually the fern's sporophyte generation. The plant consists of three main parts that include the stem, the leaves and the sporangia (the fertile region).
The stem is the main axis of the fern, which bears either roots or root-like organs for the acquisition of water and nutrients. The stem also is responsible for producing leaves or fronds. Once leaves or fronds are grown, they rarely increase in diameter and are, for the most part, not photosynthetic. Elongated stems also are called rhisomes; short and compact stems are called caudex or rootstock. Branching occurs on the rhizome or the caudex, but they may also be unbranched. Rhizomes also may be irregularly branched.