As you probably know, there are a lot of types of plants.

If you went outside right now, you could find all kinds: prickly plants, fuzzy plants, floppy plants. Plants with leaves shaped like hearts or teardrops.

Plants with flowers that are colorful, spiky, and even some that resemble monkey faces. Well, only if you live in the higher altitudes of Peru or Ecuador, for that last one. There's the banana you bought at the corner store and the ginger tea a loved one makes for you when your stomach is upset.

While many plants are good to eat, others can be downright dangerous. It’s important to know the difference. But for us humans, it’s not enough to just know about plants— we need to communicate all this information to each other: what’s different, what’s the same, what’s dangerous—or delicious— about all the plants that surround us.

After all, we don’t want anyone sipping the wrong tea. So, we’ve come up with names and categories for plants to make sense of what we learn, and to share that knowledge with others. These categories are simultaneously a super useful way of building knowledge …and totally made up.

Hi! I’m Alexis. And this is Crash Course Botany. [THEME MUSIC] There are hundreds of thousands of different types of plants.

So, what makes something, well, a plant? All the plants we know today descended from the same ancestor: a teeny-tiny green alga — that’s the singular for algae — that existed around 500 million years ago. It’s like how all people are technically related if you go back far enough in history.

What’s up, cousin? And these algae descendants all have two things in common. First, they all start as single-celled zygotes —made from a sperm cell and egg cell combo— that become multicellular embryos: the tiny beginnings of roots, stems, and leaves.

If you’ve ever looked close-up at a seed or a bean, you’ve seen a capsule that contains an embryo. Given soil, sunshine, and half a chance, it can sprout into a tiny version of a grown-up plant. Second, these things we call plants have complicated sex lives.

Plants can reproduce in different ways, and they tend to flip back and forth between them. One moment they’re swapping sex cells with each other. Another, they're releasing reproductive cells called spores.

This cycling between sexual and asexual phases is called the alternation of generations, and it helps keep plants flourishing, from Granddaddy Succulent to Great-grandbaby Succulent — and even beyond that. While these two traits unite plants into one big category, botanists need to split that category into other, smaller ones so that we can get to know the qualities of each type of plant and better understand how they're related. Imagine you collect Leafy Lovables, the hottest stuffed toys on the market.

You know each one’s name by heart: Sonny Sunflower, Molly Moss, Theo Tomato…you get the picture. And you even talk about them with other collectors, who know their Leafy Lovables by the same names. Everyone’s very impressed by your rare Ruby Rafflesia.

That’s kind of like taxonomy, the science of classifying, naming, and describing organisms. Taxonomy gives scientists all over the world a common language for talking about living things, like how two people from Japan can communicate by speaking Japanese, or two pigs can communicate using pig Latin. What?

Oh, that’s not what pig Latin is? Well, I never said I was a zoologist. Like any serious Leafy Lovables fanatic, you have a special collector’s guide to help keep tabs on your collection— where you got them, when each one was released, and which ones have been discontinued.

You might even hit up a collectors’ fair to see how your Leafy Lovables fit into the bigger history of collectible things, from Beanie Babies and Squishmallows to baseball cards and rare coins. That’s kind of like systematics, the study of how organisms are related to each other. Systematics involves studying connections between organisms alive today and those that have been extinct for millions of years.

And, sort of like organizing your stuffed plant collection, scientists use taxonomy and systematics to organize the world of living things. While systematics sheds light on organisms’ evolutionary history and relationships, taxonomy focuses more on their characteristics. Together, they help us make sense of the mass of green around us —turning chaos into order.

And they also help botanists understand how plants fit into the big picture of life. Take, for example, Sonny’s namesake, the common sunflower. It has a scientific name, too: Helianthus annuus, from the Greek words “helio” for sun and “anthos” for flower.

Its closest relatives also have Helianthus in their scientific names. And if you zoom out further, they’re placed in a bigger family that includes other, more distant relatives like daisies, dandelions…and lettuce? What, wait, is that true?

Can we see a side-by-side of the flowers? I… sort of see the resemblance. These names work like labels on life’s big filing cabinet, so botanists can organize their knowledge about plants and use it for things like understanding plants’ evolutionary history and informing conservation efforts.

And adding more labels helps build more knowledge. The Mexican-American botanist Ynés Mexía devoted years of her life to that task. Although she didn’t start her scientific career until she was in her fifties, she took off like a quiet, plant-classifying rocket.

She spent two and a half years traveling the Amazon River, identifying around five hundred plant species previously unknown to science. Today, over fifty of them are named in her honor. While the Linnaean system of taxonomy, with its particular rules for classifying and naming organisms, has become a common language for scientists all over the world, it’s more of a European invention, and certainly not the be-all and end-all of scientific classification.

Many Indigenous cultures, for example, have their own ways of classifying plants that reflect generations of first-hand experience with the organisms around them. That knowledge sometimes aligns and sometimes differs from other scientists’ categories. But when those ways of knowing join together, they can reveal patterns scientists previously overlooked.

Let’s head to the Thought Bubble… Here’s a game of Spot the Difference. At first glance, these fruits seem pretty similar. On the inside, they’re both filled with what look like ghostly white corn kernels, and on the outside, they both look kind of like Koosh balls.

They both have delightfully fruity tastes… and smell like gasoline. Unlike lemons or oranges, these fruits would not make good air fresheners. When a European botanist first described the trees that bore these fruits in the 1800s, he decided they were the same species: Artocarpus odoratissimus.

In Latin, the species name basically means, “These really smell.” Western botanists continued to think of these plants as one and the same for almost two hundred years. But Indigenous people in Borneo didn’t see them that way. They were way more familiar with these fruits, and to them, the fruits came from two completely different plants.

The Iban people knew this one as “lumok” —the cultivated tree that grew in people’s gardens, with sweeter fruit and bigger leaves. The other one was known as “pingan” — which grew wild in the jungle and had smaller leaves and fruits. The Dusun people agreed that these trees weren’t the same, and had their own names for them.

So, after consulting Indigenous people, a team of botanists, including Iban and Dusun researchers, decided to analyze the trees’ DNA. They confirmed what the Indigenous folks had been saying all along: these are two different trees! They’re relatives, but  they’re genetically distinct.

Thanks, Thought Bubble! So, as scientists learn more about organisms, their classifications can change. And when that happens, we need to adapt our language so that we can communicate more accurately.

But what even is a species, anyway? Well, just like how European and Bornean botanists disagreed about those two fruit trees, not all scientists agree on one definition of a species. Most scientists favor the Biological Species Concept – which sees a species as a group of organisms that can interbreed with each other and create fertile offspring.

Like, a tomato plant can make more tomato plants by swapping pollen with another tomato plant. But nothing happens if it swaps pollen with, say, a daffodil. They’re two different species.

This definition might seem straightforward, but there are a few problems with it. First, some organisms reproduce asexually, so they don’t need other organisms to produce offspring. Second, some organisms break the concept’s one big rule— they produce offspring with other species, or hybridize, and those hybrid species can reproduce, too.

Like, even though the London plane tree is the offspring of two different species of sycamore tree, it can reproduce. So, some say it’s better to define species by their genealogy, in what’s called the Phylogenetic Species Concept. According to this idea, all organisms in the same evolutionary lineage are one species, going back to their most recent common ancestor.

So, forget sex—it’s all about ancestry. But since it’s impossible to know everything about a species’s evolutionary history, this concept isn’t perfect, either. Another way we can think about species is as a group of organisms that has adapted to similar environmental pressures, or the Ecological Species Concept.

But even that definition doesn’t fit neatly. Many species live in more than one type of environment during their lives, like ferns that need to be around water when they reproduce and start their life cycle, but mostly live on land after that. And those are just the three most common ways to define species—there are tons more.

If all of this feels confusing to you, you’re onto something. All these definitions of a species are imperfect. After all, just like “vegetable,” “species” is just a term humans made up; plants don’t care what a vegetable or a species is.

They just do stuff. They break rules and exist just the way they are, whether they fit into our human-made boxes or not. But humans make knowledge by categorizing things, even if the categories aren’t as rigid as we’d like them to be.

So, our systems for naming and classifying are both totally made up and super useful. The word “vegetable” captures an idea: “Here’s a plant part you can eat.” The word “species” lets us say: “Here’s a group of organisms with things in common.” And then we can discuss what those things are, and use those categories to understand how nature works. So, taxonomy and systematics offer two different ways of making sense of the natural world.

And just the act of classifying and naming organisms is powerful, even when it’s imperfect. And, by the way, there’s so much more to know! There are still tons of plants that have yet to be named, classified, and understood.

Who knows what we’ll find out there? Next time, we’ll be using these tools to uncover the history of plants, starting with bryophytes and seedless vascular plants, some of the earliest plants on Earth. Hey, before we go, let’s branch out!

Mimosa mexiae was the first plant named after Ynés Mexía. What type of plant is it? Before you take a bathroom break, go find the answer in the comments!

Thanks for watching this episode of Crash Course Botany which was filmed at the Damir Ferizović Studio and made in partnership with PBS Digital Studios and Nature. If you want to help keep Crash Course free for everyone, forever, you can join our community on Patreon.