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As a SciShow viewer, you can keep building your STEM skills with a 30 day free trial and 20% off an annual premium subscription at Brilliant.org/SciShow. If you have an oak tree, and that oak tree drops an acorn, and that acorn sprouts into a seedling, that seedling is not the child of the oak tree… it’s the grandchild.

The parents lived out their entire lives in a way you never even saw. And that happens in every single land plant. Plants may seem simple, but their life cycles have a whole extra multicellular organism that’s rarely ever seen.

Here’s what they get out of this whole scheme – and where their “invisible” generations can be found. [ ♪ intro ♪] So to talk about plants it might be helpful to do a quick reminder about how animals work. Animals have two copies of their genome in each cell, which makes them diploid. During reproduction, animals make sperm and egg cells, which each only have one set of chromosomes, making them haploid.

These individual cells never really grow on their own, but they do combine to form a diploid cell again during fertilization, which then grows into a new animal. Plants saw that system and went, that’s way too easy. In plants, the haploid phase does grow on its own.

In fact, all land plants alternate between being diploid and haploid organisms in a process called alternation of generations. So here’s a quick breakdown of the cycle: Let’s start with a diploid plant, roughly equivalent to you and me, with two copies of its genome per cell. Instead of making sperm and egg cells, this plant makes haploid spores.

We call this, appropriately the sporophyte. Literally the “spore-plant”. These haploid spores then grow into a new multicellular plant, with every cell having one copy of its genome.

Eventually it produces haploid gametes – the plant version of sperm and egg cells. If sporophytes are spore-plants , this gamete-plant is, appropriately again, the gametophyte. Sometimes scientists are good at naming things.

Two gametes then find each other, fuse, and produce a new diploid organism, a new sporophyte, and the cycle begins again. Imagine that in people. Instead of producing sperm or eggs, a diploid person would just kind of spontaneously, asexually produce haploid kids all by themselves.

But then those kids would do things the normal way. How could this ever evolve? It’s actually not unique to plants.

Alternation of generations has evolved a couple of different times in the tree of life. But all plants seem to have inherited it from a common ancestor. Early in evolution, the sporophyte and gametophyte shared roughly equal halves of the life cycle, and may have looked pretty similar to each other.

In extant plants, however, one of the two forms is dominant over the other. In bryophytes such as mosses, the haploid gametophyte is the dominant form. That is, it has one copy of its genome and will make gametes, not spores.

All that green stuff you see is likely the gametophyte. The sporophyte appears only occasionally as a kind of flower-bud looking thing on a tall stalk attached to the rest of the moss. It’ll make spores and once again produce the moss you’re used to.

On the other hand, vascular plants are sporophyte dominant. That’s most land plants like trees and flowers and shrubs – and ferns. Fern gametophytes look like little green splats.

They’re free-living enough to be noticeable, if you’re looking hard enough. However, flowering plants, as well as conifers, gingkos, and the like, have become so lopsided that you have to essentially dissect plant bits to find the gametophyte. And this is where the invisible generations come in.

The fully-grown gametophyte is just a couple of cells producing the pollen cell or ovum, embedded in different structures in the flower and dependent on its quote-unquote parent to survive. It’s still there, as its own multicellular thing. Though we’re getting to pretty charitable definitions of “multicellular,” like, single digits.

And it produces the pollen and egg cells the plant needs to continue the alternation of generations. That’s why our oak seedling from the beginning is the grandchild of the big oak tree. Its immediate parent lived and died unnoticed in the oak flowers.

But… why though? What advantage does this give to plants? It might allow plants, and other similar organisms, to take advantage of both the reliability of asexual reproduction and the diversity-generating process of sexual reproduction.

When you reproduce asexually, you don’t need a partner to get the job done. But when you recombine your genes through sexual reproduction, you’re a lot more resistant to any surprises evolution might throw your way. It also allows plants to double-dip into the advantages of both diploid and haploid genomes.

Diploid plants may have an easier time compensating mutations because they’ve essentially got a backup copy. At the same time, for the same reason, the haploid phase might be really good at weeding out problem mutations. As to why plants grew to favor one or the other so dramatically, instead of having a more even split, it looks like this is something scientists are still trying to figure out and debating over, but we think that with vascular plants at least it might have been a key part of how plants took over dry land.

One idea is that, in early plants, like in modern mosses and bryophytes, the gametes needed abundant water to survive and find each other. This limited where they could grow on land. But the haploid spores didn’t need to find other spores – they could just grow on their own, even during drier times.

The diploid genome – with its diversity and ability to have back-up genes – may have had an easier time adapting to dry land. Finally, some studies have suggested there may have actually been something genetic about the sporophyte stage that enabled plants to evolve vascular tissue in the first place. That’s the nutrient-carrying tissue that allows plants to grow up and away from the water.

This means gametophytes would have always have been doomed to stay low and water-dependent: they never co-opted their genetic machinery the same way their children did. If that’s true, it’d make sense why the sporophyte-dominant plants took over the land. But like I said, it’s still an open question.

We, that is to say, humans, can also take advantage of these generational shenanigans. There’s a way that scientists can take pollen and stress it out until it actually starts to grow into an embryo without ever meeting an egg cell. Chemicals can then be used to force it to double its chromosomes.

Basically, you they skip a generation and create a quick pseudo-clone of the parent plant. Which is really neat if you’re into plant breeding because it means you can develop new breeds very quickly without having to mess around with all the pedigree stuff if it had to reproduce sexually. So there you have it.

Unlike animals, plants go through half their life cycles with half as much DNA, and they swap the way they reproduce every time. Evolution has reduced the role of some of those halves, but it hasn’t gone back on the process entirely, maybe just because evolution doesn’t really do that kind of thing. But while we can turn the system to our advantage, it’s also just really interesting to think about these whole generations of plants that we, for the most part, never notice.

So much of life happens kind of behind-the-scenes or in ways that we don’t know about. So I hope you’ve enjoyed this little peek at the invisible. Sometimes what’s going on behind the scenes is the coolest part of the process.

And you can learn about what’s going on behind the scenes of the device you’re watching this video on thanks to the Brilliant course Algorithms and Data Structures. Brilliant is an online learning platform with thousands of lessons in computer science, science, and math. And this computer science course is a nice mid-level foundation for the aspiring computer programmer to keep building on.

After you’ve taken Brilliant’s Introduction to Algorithms course, this one teaches you how to store and manipulate data, with helpful pointers along the way. We definitely wouldn’t get to talk with you about cool science the way we are right now without all of those algorithms running in the background. So you can learn more about them at Brilliant.org/SciShow or in the link in the description down below.

That link also gives you a free 30 day trial and 20% off an annual premium Brilliant subscription. Thanks to Brilliant for supporting this SciShow video, and thank you for watching! [ ♪ OUTRO ♪]