All land plants, and some algae, have life cycles in which a haploid gametophyte generation alternates with a diploid sporophyte, the generation of a plant or alga that has a double set of chromosomes.
The haploid gametophyte phase is free-living and photosynthetic. The diploid sporophyte grows from and is nourished by the gametophyte. Images from Purves et al.
The 2n zygote germinates and grows to become the sporophyte. Meiosis occurs in certain of the cells in the sporophyte, producing haploid swimming spores that will settle to the ocean floor and produce the next generation haploid gametophyte stage.
 See also: Species, Sperm, Meiosis, Plant, Sporophyte
  
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