"Deep Look", PBS's ultra-HD short video series, which I wrote about here last year, has a holiday-themed installment that I thought was well-worth sharing.

One small technical error I noted: the narrator says "Seeds have to be exposed to catch pollen and be fertilized” … actually, the eggs, or ovules do. Before they are fertilized they are not seeds.

Those seeds tucked inside the cone are the source of “pine nuts” in certain pine trees that pack a lot of snacks for the road into seeds.

IIf you're happy with this explanation, which is correct as far as it goes, stop reading here. But the actual situation with Christmas tree sex is a lot more complicated -- and weird -- than "Deep Look" lets on. That's because conifers -- like all plants -- actually have an entirely separate lifeform/phase that animals lack: the gametophyte.

What is a gametophyte? Well, every cell in your body except your eggs or sperm is diploid – that is, each cell contains two entire copies of human DNA, one set from your mother, and one from your father. You may recall this from high school biology when the teacher emphasized how all 23 of our chromosomes come in pairs.

Normally, in your ovaries or testes, these diploid cells undergo meiosis -- reductive cell division, another one of those annoying terms the teacher kept harping on -- to get down to a single set of chromosomes in eggs and sperm. That way, when egg meets sperm, the resulting zygote ends up with two entire sets of chromosomes again (and not four).

Now imagine that, instead of making haploid (single DNA copy) eggs and sperm, you made haploid cells called spores instead. Rather than fusing with another spore to re-establish diploidy, these cells start dividing via the usual method (mitosis) and grow into an entirely separate form of human. The human doppelganger – which, if it were like plants, wouldn’t look anything like a human and may or may not be free living(!) -- would then go on to make the eggs and sperm. That’s a gametophyte.

In more early-evolved plants like mosses and ferns, the gametophyte is a large, visible, and free-living organism. The fern gametophyte is a separate little plant that lives on its own in the forest undergrowth. In more recently evolved plants like conifers, the female gametophyte is small and inconspicuous mass of cells hidden inside the female cone of the sporophyte, the large diploid plant we see. It will produce an egg to be fertilized, but the rest of the cells of the gametophyte divide to become food for the embryo that will be the result of fertilization.

When you eat a pine nut, most of what you are eating is the female gametophyte. There is a little plant embryo hidden inside -- if you cut the nut in half, you will see it tucked in there all cute and innocent just as you would if you split a peanut in half -- and it is diploid. Along with the red alga Porphyra which makes the nori wrappers that enfold sushi, the pine nut is probably the only haploid food that humans routinely eat.

The male gametophyte is only four cells big, but it gets a level of exposure and a big adventure that the female gametophyte can only dream about. It is the male pollen grain, the little winged structure that you saw at the end of the video. Inside it is a cell that will one day divide to make two sperm nuclei (conifer sperm don't just lack tails, they don't even have their own separate cells), which then travel down the pollen tube created by the other cell and into the female gametophyte -- should they be lucky enough to land on a female cone and not, say, on your freshly washed car.

If you love life cycle diagrams like I do, here is a nice one that sums up the admittedly confusing -- and perhaps surprisingly complex -- sex lives of Christmas trees.