I found the above to be very insightful. Beware of the ads, though; as in, there are a lot of them (but if you search for articles on your typical search engine, it's probably something you're used to).
Apparently, some plants produce gametes some generations and spores other generations (and each of the two generations takes a different form, with one being haploid and the other diploid). Cool stuff. This includes at least pine trees, mosses, ferns, and hornwort (according to what the article says).