Fungi, Slime Molds, Algae
Land plants descended from algae, many of which exhibit alternation of generations (AoG). Fungi and slime molds also practice AoG.
Fungal mycelia are typically haploid. Each mycelium has a sexual orientation. When mycelia that may mate meet, they produce a pair of multinucleate cells which fuse to form diploid nuclei, in a process termed karyogamy.
Karyogamy produces a diploid zygote. This short-lived sporophyte soon undergoes meiosis, forming haploid spores. The spores germinate, developing into new mycelia.
Slime mold AoG is similar to fungal mode. Haploid spores germinate into myxamoebae: swarm cells that find each other and create a colony.
Swarm cells fuse to form a diploid zygote, which develops into a plasmodium. A plasmodium matures, whereupon producing 1 or more fruiting bodies (depending upon species) which shed haploid spores.