Meet Barbacenia. Plants that grow between a rock
and a hard place… literally. They’ve got hairy roots that
ooze rock-dissolving acids, letting them chisel their way
into Brazilian mountaintops. So, instead of getting nutrients
from the soil like most plants, they’re busy carving into boulders. Now, the petunias in your grandma’s yard aren’t
out there dissolving stones to make a living. But the rest of the plant
world is no less hardcore. Plants transport water against gravity, defend
themselves against predators, and all-in-all function through super-specialized,
interlocking systems in their bodies. Basically, the closer you look at plants,
the more mind-blowing they become. Hi, I’m Dr. Sammy, your friendly
neighborhood entomologist, and this is Crash Course Biology.
Speaking of mindblowers, Callieeeeeeeee, drop that theme music, please! We’re
dropping theme music, not our manners. [THEME MUSIC] Plants are wildly diverse—there
are nearly 400 thousand species, and that’s just the ones we know of. So, today, we’re zooming in on one category:
angiosperms, which grow flowers and fruits. But don’t worry, you can go check out Crash Course
Botany to learn about our other plant friends. I may be biased, but I highly recommend
it. And sorry, ferns, we still love you. You see, angiosperms are the
dominant plant life on Earth. Almost every plant you would
recognize is an angiosperm, whether it’s a tomato, oak tree, or wildflower. And they grow basically everywhere on
Earth, including the Arctic tundra. Now, just like our bodies need
systems to help them eat, breathe, poop, fight germs, and so on, plants rely
on integrated systems in their bodies. Like, you might have heard of photosynthesis, the process plants use to turn sunshine, carbon
dioxide from the air, and water from the soil, into sugars for the plant’s food
and basic building material. But it’s not a one-and-done:
photosynthesize and chill. Plants then need to move those sugars throughout
their bodies to keep things running smoothly. [Chapter 3 - How plants transport nutrients]
For this, angiosperms use veins called phloem. You can actually see phloem in those
stringy things on peeled bananas. Though in most cases, they’re inside
the plant where you can’t see them, like here in this redwood tree. The phloem moves sugars from the
leaves to the rest of the plant. This can happen either through diffusion, where molecules passively spread into an
area where there aren’t a lot of them. Or, through active transport, where the
plant uses energy and special proteins to drag molecules to where they need to go,
even if there’s already a bunch there. But let’s back up a second. For photosynthesis to even begin, plants need to move stuff in a totally
different, way more impressive direction. You see, they have to suck water
from the soil, into the roots, and upwards to get to the leaves,
where photosynthesis happens. That means plants are working against gravity. Now, this isn’t a huge issue for
a low-lying plant like a moss. But for an angiosperm like
a 100-meter-tall redwood? It has to get that water all the
way to the leaves at the very top. I don’t know about you, but I’ve
never seen a waterfall go in reverse. Keeping those sky-high leaves hydrated is a
bit of a journey, and it begins at the roots. Roots, by the way, are one of the main types
of plant organs, along with stems and leaves. An organ is just a structure made of
tissues working together to do a job. And tissues are just groups of cells working
together, so organs are like a system of teams. Plants —and animals for that matter — are all
pretty much organized into these kinds of systems. But back to defying gravity. Roots need to absorb lots of water,
so most angiosperm roots have a bunch of little hairs to suck up as much as
possible through many different entryways. Again, this transportation
happens through diffusion: If there’s more water in the soil than the roots,
the H2O will passively travel into the plant. Meanwhile, nutrients like salts
and nitrogen may get into the roots through either diffusion or through
active transport, which requires energy. But that just gets us as far as the roots! From there, the solution of water and
minerals heads to another type of plant vein, called xylem, which will carry
everything to the rest of the plant. But again, that usually means going up. So, how does that work? The first thing to know is that water may
not be immune to gravity, but it is sticky. On the molecular level, water molecules
are slightly attracted to each other, so they tend to group up like
the cliques in a teen movie. That’s called cohesion. And when water molecules are gently
attracted to other things — like, say, the walls of xylem — that’s adhesion. The second thing to know? Water may come into a plant through
the roots, but if it’s not used for photosynthesis, it leaves through… well,
the leaves in the form of water vapor. That’s called transpiration, and it’s
yet another process powered by diffusion. When you put cohesion, adhesion,
and transpiration together, it’s a more magical trio than
peanut butter, jelly, and bread. [Sammy sings “Peanut Butter Jelly”] Sorry. Ahem. As water - in the form of vapor
- transpires out of the leaves, those sticky water molecules pull
on the molecules behind them. And those molecules pull on the ones behind them. And suddenly, there’s this giant conga
line that drags water all the way from the soil to the roots to the leaves
at the tops of the tallest trees. Take that, gravity! So, that was a quick and dirty tour of some of plants’ resource acquisition
and nutrient transport systems. But, just like us, plants don’t only need to
eat stuff and move those nutrients around. They also need to get rid of stuff. And no, I’m not saying plants poop. I’m just saying, plants… expel waste products. Which, yes, sounds like a fancy
phrase for poop, but it’s not! Like, take transpiration again. A huge amount of water that’s no longer needed
can actually leave a plant through transpiration. We’re talking enough water for a
forest to create its own weather. See, in the Amazon rainforest, it rains. A lot. Typically, rainy seasons are caused by
seasonal winds carrying moist ocean air inland. But the rainy season in the
Amazon actually starts two to three months before that — thanks to local trees. Scientists estimate there are nearly 400
billion trees in the Amazon — about 50 for every person on Earth! — and all of them are releasing
water from their leaves on a daily basis. Enough that it collects in the atmosphere
and condenses to form rain clouds! As the clouds dump rain onto the
forest, this raises the humidity, which in turn warms up the
atmosphere just a little. This causes air to rise and start circulating,
like the bubbles at the bottom of a teapot. This circulation then causes the local wind
patterns to change and starts dragging in wet air from the ocean and kicking off
the rainy season months in advance. It goes to show how big of a deal
transpiration is — and how adding or removing trees from an area can
do more than just shape the obvious parts of an ecosystem: On a big enough
scale, it can even change the weather. So, leftover water transpires out of plants
through pores in their leaves called stomata. But the stomata are also where carbon
dioxide and oxygen enter and leave the plant. You guessed it, through diffusion. And for plants, as for people, maintaining the right balance of stuff
we need vs. stuff we don’t can be tricky. For every molecule of carbon dioxide
a plant takes in through its stomata, it can lose around 400 water molecules
that it may want to hang onto. It needs that water not only for photosynthesis, but because water helps plants
keep their structure and shape. So, to maintain the right balance,
plants have evolved strategies for how to take in air through their
stomata without losing too much water. For example, cacti only open their
stomata at night when the air is cool; if they opened them in the daytime, they
would lose a lot more water in the heat. OK, we’ve seen lots of
interlocking systems at this point. Plant systems are taking in resources,
moving ‘em around, and getting rid of waste. Check, check, and check. But how do plants get rid of
other things… like predators? Yes, you adorable bunny you, you are
a threat to my strawberry plants. Rooted in just one spot, plants
can’t just up and book it when lil thumper comes wiggling his
little nose in their direction. So to fight against local predators, plants
might have stabby thorns on their stems, or fuzzy hairs that make it harder for
very hungry caterpillars to reach a leaf. Some plants even use chemical weapons —
special compounds that make them taste awful, or that make them poisonous to would-be diners. And they ramp up production of those nasty
compounds when they sense plant-eaters are around. Plants can warn each other of danger, too. For instance, when a giraffe checks into
the Acacia tree buffet, the tree releases chemicals on the wind that warns its
buddies that leaf-eaters are in the area. Any Acacia that gets the message
starts producing gross compounds. So, ultimately, giraffes have to go upwind to
find tasty trees that haven’t gotten the memo. And they can even call for help. Some plants, like tomatoes, detect compounds
from the saliva of a caterpillar and release a chemical signal which summons the
caterpillar’s worst enemy: a parasitoid wasp. This wasp stings the caterpillar paralyzing it and allows its babies to chow
down on the nutritious meal. And when it comes to microscopic
threats like bacteria, viruses, or fungal infections, plants have immune
systems to keep themselves safe, too. Their cells have proteins and special molecules
that can recognize and neutralize invaders. So bacteria and bunnies don’t stand a chance, usually…there’s no fool-proof defense
in the evolutionary arms race. As the armor grows thicker,
the swords grow sharper still. Now, all of that said, for any of
the systems we’ve mentioned to work, plant cells have to be
communicating with each other: If each plant part were just vibing on its own, they wouldn’t know how to coordinate
to take down a germ or make plant food. Plant cells get information about the
outside world from receptor proteins, which change in response
to the plant’s environment. Like, a protein that’s sensitive to light will change shape when the sun
comes up and when it goes down. And that can cause a chain reaction that
leads to bigger changes in the plant — like, cueing a cactus that it’s
safe to open its stomata. Plants can also coordinate their bodies
with hormones, just like humans do. These signaling molecules travel around and
trigger responses in any cell they can bind to. And there are more varieties of
hormones than there are flavors of jelly beans in one of those
giant jugs you win in a raffle. Some hormones, for example, help plants reproduce. Like, check out these strawberry plants. The flowers are their reproductive organs. In fact, that’s true for all angiosperms! Which does make giving your crush a bouquet of
roses a little weird…but at least they’re pretty. Anyway, when the plant is ready to reproduce, the hormone florigen lets flowers
know they should start blooming. That yellow, powdery stuff called
pollen contains plant sperm cells, and the hormone auxin helps it develop. When that pollen hits an egg
in another part of the flower, it’s only a matter of time until seeds
and baby strawberry plants are on the way. But plants reproduce in other ways, too. Some essentially make clones of themselves,
in a process known as asexual reproduction. And some plants do both! Like strawberry plants, for instance. And there are pros and cons
to both types of reproduction. Asexual reproduction is generally simpler: it
doesn’t require the organism to find a mate. But sexual reproduction can also introduce
genetic diversity into a population, which helps a strawberry patch survive long-term. So, being able to reproduce both ways
gives plants the best of both worlds. So, while it can seem like
plants are just chilling, minding their own business… there’s
a lot more going on than we realize. Our leafy green neighbors are constantly
moving nutrients throughout their bodies, defending themselves against danger, and communicating within and among themselves
to keep these overlapping systems functioning. And in the end, plant life doesn’t
just make our world more beautiful: It can shape the weather and climate, act as important sources of food
and medicine, and reshape our world. Next time, we’ll be jumping into the
amazingly complex world of animals, starting with how they get the stuff they
need and get rid of the stuff they don’t. I’ll see you then. Peace! This series was produced in
collaboration with HHMI BioInteractive. If you’re an educator, visit
BioInteractive.org/crashcourse for classroom resources and professional development
related to the topics covered in this course. Thanks for watching this episode of Crash
Course Biology which was filmed at our studio in Indianapolis, Indiana, and was made
with the help of all these nice people. If you want to help keep Crash
Course free for everyone, forever, you can join our community on Patreon.