It has been argued that there is an important evolutionary advantages to the alternation of generations plant life-cycle. Theoretical advantages of alternation of generations
In the early stages of growth, the sporophyte grows out of the prothallus, depending on it for water supply and nutrition, but develops into a new independent fern, which will produce new spores that will grow into new prothallia etc., thus completing the life cycle of the organism. The sperm are able to swim to the ova for fertilization to form a diploid zygote which divides by mitosis to form a multicellular sporophyte. The prothallus develops independently for several weeks it grows sex organs that produce ova ( archegonia) and flagellated sperm ( antheridia). The haploid spores fall from the sporophyte and germinate by mitosis, given the right conditions, into the gametophyte stage, the prothallus.
The fully grown sporophyte, what is commonly referred to as the fern, produces genetically unique spores in the sori by meiosis. Spore-bearing plants, like all plants, go through a life-cycle of alternation of generations. Prothallus of the tree fern Dicksonia antarctica (note new moss plants for scale)
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