Ferns are one of the oldest plants in existence. On the evolutionary scale they arrived after bryophytes but before gymnosperms. The physiology is different among the three but the biggest difference is reproduction. Bryophytes and ferns reproduce via spores, whereas gymnosperms use seeds. Most ferns are homosporous but a small percentage (1 percent) is heterosporous.
Ferns have leaf-like structures called fronds. The fronds are made up of feathery parts called pinnae. Ferns reproduce by spores instead of seeds. The spores are carried under the pinnae and are released when they are ready. Wind or animal movements will carry them to a hospitable spot where they become a gametophyte. This body has both male and female cells and if conditions are right and there is moisture in the air, the two cells swim towards each other. The union creates a sporophyte that in turn will grow into an adult fern.
In homeosporic reproduction, the gametophyte that was produced is bisexual since it has both male and female organs. Yet it is not a true production of the adult fern until the two organs meet. The organs are called isospores. The gametophyte will eventually produce antheridia or sperm bearing structures and archegonia--the egg-bearing structures.
The production of two separate sex gametophyes is called heterosporic reproduction. The spores that create the gametophye are different. The male spore that produces sperm is the microspore and the female spore is the megaspore. The male produces a microgametaphyte and the female produces a megagametophye. Fertilization takes place forming a zygote and then an embryo of the fern.
The change to heterosporic reproduction is seen by scientists to be an evolutionary swing to the seed. The process affords more control over reproduction and a better chance of fertilization. The separation of the male and female gametophytes points toward another theme of evolutionary advancement--division of labor.
The spores in the heterosporous are two sizes; the microspores and the megaspores. The usefulness of the smaller spore is its ease of transportation and the larger one makes it easier for the microspore to find. The likelihood of passing on half the complete set of DNA with two parent spores increases versus having a full complete set in a homosporous relationship.