Alternation of generations
During the Devonian, several plant groups independently evolved heterospory and later endospory, in which single megaspores were kept inside the sporangia of the parent sporophyte. These endosporic megaspores had a miniature multicellular female gametophyte with female sex organs and egg cells. The ova were fertilised by free-swimming sperm produced by windborne miniaturised male gametophytes in the form of pre-pollen.