Dear Dr. Universe
How are plants considered living if they don’t have a heart or brain?
— Lily, Pennsylvania
You and I are both animals with backbones, so we have lots in common. Our ways of being alive look similar. We have hearts to pump blood. We have brains that help us think and communicate.
But plants don’t have the same body systems we do.
I talked about your question with my friend Michael Knoblauch. He’s a plant scientist at Washington State University.
He told me plants are seriously good at being alive. In fact, 80% of all the living things on Earth are plants.
“Go out in the forest and look around,” Knoblauch said. “You might see birds and insects. But what you really see are plants. The most successful organisms on this planet are plants, even without a heart or brain.”
But it can be tricky to figure out what counts as a living thing. Here are the traits living things share, according to scientists who search for signs of life.
Living things reproduce: Flowering plants make baby plants when pollen from one part of the flower — called the stamen — falls onto another part of the flower — called the pistil. In some plants, that happens in the same flower. In other plants, pollinators like the wind or insects move pollen from one flower to another. That makes a seed that can grow into a new plant.
Living things grow and develop: If you peek inside a seed, you’ll see the start of a baby plant—called an embryo. When that seed sprouts, its roots grow down, and its shoots grow up. Just like you, the plant will use energy to grow until it’s mature.
Living things respond to their environment: A plant’s roots and shoots know which way to grow because the plant can sense gravity. It responds by growing away from gravity or toward it — called tropism. Plants also sense light, water and touch. They automatically grow toward or away from those things. It’s like how your body automatically senses and responds to light by shrinking or expanding the pupils of your eyes.
Living things are made of cells: Some living things are just one cell. But humans and big trees have trillions of cells. Plant cells can be larger and more cube-shaped than human cells. They also have other differences — like a rigid outer layer called a cell wall and a big storage compartment called a vacuole.
Living things take in and use energy: Plants collect energy from the sun. Then, they change it into energy they can use through photosynthesis. When you chow down on veggies, your cells take in the energy stored in the plant.
Living things keep their inside environments steady: Plants maintain a steady, internal temperature by releasing water — a process called transpiration. As the water evaporates, it takes heat with it. It’s kind of like how you sweat to keep your body cool.
Living things adapt to their environments: Plants haven’t always had flowers. Millions of years ago, some plant leaves started growing around the plant’s seeds. That helped those plants survive. They passed on their weird leaves to their baby plants. Eventually, those leaves became flowers. It’s like how your ancestors’ hands slowly adapted so you have flat fingernails instead of claws like me. It turns out we share some unbe-leaf-able similarities with plants after all.
Have a science question? Ask Dr. Wendy Sue Universe, WSU’s resident science cat and writer, by email at Dr.Universe@wsu.edu, on her website at askDrUniverse.wsu.edu, via Twitter at @AskDrUniverse or at facebook.com/AskDrUniverse.
