Slight NSFW warning, I guess. I mean, unless you’re reading this from the Victorian era (and, if so, please tell me how), or are actually a plant (again, inquiring minds want to know), the subject itself isn’t too shocking, but I will use some somewhat indelicate language occasionally. And I’ll be using the word sex. Like, a lot.
While animals have an exceedingly broad range of ways to have sex, most actual reproduction among animals is more or less the same. There are a few outliers–haploid male bees, parthenogenesis, budding, and so on–but, for the most part, once they have sex, however they do so, sperm meets egg, an embryo develops, then momma either lays the egg somewhere, or keeps it inside her body until it’s ready to be born.
But plants are… not so simple and straightforward, by a *long* shot.
The most obvious difference is that, unlike virtually every land animal, plants are all basically sessile. So they can’t have sex directly. Unless they self fertilize, they either need a pollinator, such as a bee, to carry their gametes from the male to the female, or they just sort of spooge male gametes into the wind and hope for the best.
So, pollen is basically sperm and seeds are basically eggs, right? Not quite. This is where the weirdness that is alternation of generations comes in. See, unlike animals, which are (barring certain insects, as previously mentioned) diploid whenever they’re multicellular, with their haploid stage only consisting of single-celled gametes, diploid plants produce haploid spores *which become multicellular* before producing gametes. It would be kind of like if, instead of having sex to make a baby, your sperm and eggs became little babies on their own, which then went off to have sex and make your actual baby.
A few terminology notes, to help you keep everything straight. Haploid=has only one set of chromosomes, diploid=has two full sets, often written as “n” and “2n”. Some organisms, especially certain plants, are tetraploid (ie 4 copies, or 4n) or whatever, but for our purposes we’ll treat every organism like it can either have one full set or two, whether that “full set” consists of one copy of each chromosome or more than one. And a gamete will always be haploid, and will combine with another gamete to produce a new organism. A spore can be either haploid or diploid, and grows directly into a new organism on its own.
Now, a lot of algae have either approximately the same form whether they’re haploid or diploid (such as sea lettuce), or have different but roughly equal haploid and diploid generations (such as sea pearls). Some red algae go all in on this “different forms” business, having 3 distinct phases, often a gametophyte (haploid) “seaweed” phase that produces haploid gametes, a sporophyte (diploid) phase that grows on the gametophyte and produces diploid spores, and a separate sporophyte phase (often an encrusting algae, living in clam shells and the like) that produces haploid spores.
Land plants tend to be a little bit simpler. For bryophytes, such as mosses, the dominant phase is the gametophyte, and the sporophyte is usually physically dependent on the gametophyte. That means the fuzzy looking green stuff that we think of as moss is mostly haploid. Modern vascular plants tend to be the other way around, with the big green part being diploid, and only pollen (the male gametophyte) and seeds (the female gametophyte) being haploid. But they’re still multicellular (if rather less so, pollen typically only has a few cells).
There is more weirdness to come, especially when you add fruit to the equation, but that will be for a future post.
Science is Really Weird 