Welcome back to Nature League!

So far on this channel, we’ve explored several topics related to biodiversity, and spent a month checking out invertebrates, which make up close to 95% of all animal species on Earth. But what about species that aren’t animals?

It’s time to zoom out from the animal kingdom and check out another kingdom of life on Earth. This month is all about plants. [CHEERY INTRO THEME]. Imagine a place on the surface of Earth.

Unless you thought of a certain polar region or some very harsh deserts or mountaintops, that place contains some plants. And plants are more than just widespread. They’re one of the main reasons that other species are widespread too.

Plants not only provide oxygen, but serve as the ultimate food source for most animals on land. But if we’re going to talk about plants, there are so many more things to discuss than just what they do for other organisms. Plants are incredible in their own right, and possess so many stunning adaptations and life histories that we could never begin to detail them all.

So, for this Plant themed Lesson Plan, we’ll discuss some basic and advanced aspects of this remarkable group of organisms. Let’s start with the origin story. Most lines of evidence suggest that land plants are most closely related to green algae, and that both most likely evolved from a common ancestral alga.

The first plants were most likely small and unicellular, much like their closest algae relatives. However, there are currently close to 300,000 known species of plants on Earth, and these now include all kinds of shapes, sizes, and varieties. Let’s check out the diversity of plants by looking at one currently proposed phylogeny, or evolution and relationships of a group of species.

The first split on the plant phylogenetic tree is between vascular and nonvascular plants. Nonvascular plants lack extensive developed tissue systems that help move water and nutrients around the plant. These are commonly called bryophytes, and include liverworts, mosses, and hornworts.

Vascular plants make up all the rest. In fact, close to 93% of all known plant species are vascular. These species can either be seedless or seeded, and that’s the next big split in the tree.

Seedless vascular plants include lycophytes, which include club mosses, and monilophytes, which include ferns. So our last, and largest, group of plant species are vascular and have seeds- but! Not all seeds are created equal.

The last big split in the plant phylogeny has to do with the type of seed- specifically, whether or not the seed has chambers during development. The two groups of seed plants are gymnosperms and angiosperms. Gymnosperms have seeds that are not enclosed in chambers.

In fact, the prefix “gymnos” means naked. So yep - naked sperm it is. Angiosperms have seeds that develop inside of chambers, and these chambers typically come from flowers.

So, angiosperms are also referred to as “flowering plants”, and they make up close to 90% of all plant species. Now that we’ve learned a bit about how different plants are related to one another, it’s time to dive into one of my favorite things about any group of organisms- their adaptations to life on Earth. Back in the day of that ancestor algae, at least one population accumulated certain traits that allowed individuals to live on land instead of in the water.

These populations would eventually evolve into the first land plants. And just imagine what that transition would be like if you were a photosynthetic organism. Underwater, the sunlight is filtered by things like plankton and the water itself.

But on land, these starter plants could enjoy the sun undiluted and pure, getting way more than was possible underwater. Life on the sunny side of the street came with some disadvantages, however. Being out of the water meant that obtaining water became more of a challenge.

Also, living on land means living with more gravity, and that put extra stress on plants’ external and internal structures. The difficulties of living on land put selective pressure on newly arising mutations, which over time led to some of the incredible adaptations we see in the plant kingdom. Let’s check out some of these plant adaptations for life on land.

The move onto land meant that plants were permanently exposed to the air and drying out was a major concern. And it’s not like plants could just grab some lotion...except they kinda did. Many plants evolved a covering called the cuticle, which contains wax.

That wax compound is hydrophobic, meaning it doesn’t dissolve in water, so the water is kept inside. Brilliant, right? Not only did they figure out waterproofing, but plants ended up with another special adaptation on their outer layer.

Most plants have special pores called stomata. These openings allow the plant to exchange things like oxygen and carbon dioxide with the air. But water can evaporate from stomata, and that can lead to dehydration.

Not to fear though. Stomata have special mechanisms that allow them to actually close when it’s hot outside. Nice one, plants.

And while land plants have come a long way, the earliest versions didn’t exactly have fully formed roots and leaves...and this meant trouble in terms of getting nutrients. So, instead of a physical adaptation, why not a behavioral one? Why not, say, phone a friend?

That’s exactly what some of these early plants did. They teamed up special fungal networks called mycorrhizae to get nutrients transferred from the soil to the plant. Teamwork made the dream work - and for plants, that dream was eating, surviving, and colonizing land.

Although these adaptations absolutely helped ancestral plants make the move from water to land, the big game changers didn’t appear until seeded plants. These adaptations are seeds and pollen grains. So what’s the big deal?

Well, plants aren’t quite known for their movement ability, and that meant things had to get creative when it came to sex. Seedless plants like ferns have sex cells that use water as protection- this is one reason these species typically live in wet environments. With seed plants, the female sex cells are inside ovules, and the male sex cells are inside of pollen grains.

And unlike the swimming sperm of seedless plants, pollen grains can be carried on the wind or by other organisms, meaning that plant sex didn’t have to rely on having water around. Seeds themselves develop after fertilization, and they consist of the embryo, a food supply, and a protective coating. So not only did the evolution of seeds mean less reliance on water, but it meant that seed plant embryos had way more protection, a nicely packed lunch, and the ability to stay dormant for days, months, or even years before emerging.

As cool as seeds are, the most famous reproductive adaptations of plants belong to the angiosperms. That’s right, we’re talking about flowers and fruits. Flowers are unique structures made up of up to four kinds of modified leaves.

Flowers allow the transfer of pollen from one individual to another in a way that is less random than just blowing in the wind. The different types of modified leaves that can make up flowers have different roles that aid in sexual reproduction. For example, petals are brightly colored, and this serves as a way to attract pollinators.

So what about fruit? Angiosperm seeds have extra protection in the form of a thickened ovary wall. The ovary matures into what we call a fruit.

Fruits don’t just protect seeds, but they also help them get dispersed. This can be done in several ways, and if you’ve ever eaten fruit, you’ve been a part of one of those mechanisms. Amazing adaptations and unique reproduction aside, one of the coolest things about plants as a group is that they are truly creatures of two worlds.

Think about it: unlike almost any other life on Earth, plants are simultaneously living above and under the ground. The parts of the plant above ground take in things like carbon dioxide, whereas the parts underground take in nutrients. The entire plant concept depends on living in both environments.

Which is insane when you consider that plants are so well known for their relationship to the sunlight. All of this, and we haven’t even touched on the craziest thing. Plants eat sunlight and carbon dioxide, and poop oxygen.

What even is that?! If you ever want to feel like a slacker, just compare yourself to what plants are doing just by “pooping”. Plants are absolutely amazing, and we look forward to exploring this theme throughout the month.

Thank you for watching this episode of Nature League. Make sure to subscribe at youtube.com/natureleague, and we’ll see you next week for a plant themed Field Trip. Nature League is a Complexly production.

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