I think an alien looking at life on earth might find it very peculiar that so many of our life forms split into 2 independent parts that are unable to reproduce unless they match up with a partner of the opposite "sex". This is no stranger than that -- but it's fascinating to us who haven't encountered it before. The "sex" thing turns out to have big advantages: mostly that it allows for faster evolution by allowing…
Might be strange that we have sexes. Might also be strange that we only have 2 sexes. Many plants have two different generational types (haploid and diploid). Meaning each "generation" alternates between two different organism types. Fungi have both sexual and assexual reproduction [2]. But their sexual reproduction occurs when two cells directly fuse and then undergo meiosis into haploid spores instead of a diploid…
For the vast majority of plants, these are not physically separate organisms. The key difference here is that the haploid cells of the plant can themselves undergo mitosis, which animal gametes cannot do. Since we generally consider cells with different genetics to be different individuals, we speak of plants as having alternation of generations.
For the most part however, the haploid cells live in the diploid organism or vice versa, depending on which branch of the kingdom you're considering. It's not like botanists run tests on a particular specimen to determine its ploidy. For any given species, we know what the ploidy of the individual is. For example, flowering plants are all diploid. If you find a flowering plant, the individual has two sets of chromosomes, but somewhere in its tissues there is a haploid cell (descended from the parent) that is undergoing meiosis, and is thus a separate organism.