Alternation of generation refers to life cycles where organisms have two different multicellular stages: one that is diploid, one that is haploid. This is different from us. Human beings at any multicellular stage, are diploid. The only haploid stage we have is sperm and eggs - single cells. Gametophytes are the haploid stages of plants. It's the part that makes the gametes (sperm & eggs). In some plants, the gametophyte is the dominant stage, like in mosses. The green stuff you imagine when you think of a moss is the haploid stage. There are males that make sperm and females that make eggs. In most plants, however, the gametophyte stage is very small. In larger plants, the pollen is the male gametophyte. It contains cells that make sperm. The female gametophyte is the ovule and is inside the ovary in the flower or cone. But, even though they are small, they are still multicellular. The sporophyte stage is what is produced after fertilization and is therefore diploid. So, the actual plant or tree that you think of is the sporophyte. The sporophyte makes haploid spores by meiosis that become the gametophyte. The gametophyte then grows and develops and makes the gametes by mitosis which, when fused through fertilization, make a new sporophyte.
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