EAST LANSING, Mich. — “Why do plants grow straight?” asks Sara H., age 5, New Paltz, New York, in the most recent edition of The Conversation’s “Curious Kids” column. Professor Beronda L. Montgomery has the answer, below.
Have you ever been at a sporting event or concert and had to wiggle and reposition to get in just the right spot to see the action? Maybe you needed to shift left or right to see between two people. Perhaps you even had to squat on your seat to see over the person in front of you.
Well, plants often have to do something similar so that they can “see” as much light as possible. Plants need light to perform photosynthesis – making sugars from water and carbon dioxide in the air to feed themselves.
If sunlight is directly above them, then plants will grow straight up toward it.
Sometimes, though, it’s not that simple. For example, you might have seen house plants bending towards a window rather than growing straight and tall. When light comes from an angle, plants will curve toward it to get better access to the light they need to grow. Hormones in the plant’s tissues, called auxins, make cells on the dark side of the plant grow taller, bending the plant toward the light.
In a forest, plants may branch out so that their leaves are in open patches of sun, rather than in the shade. This often happens if taller bushes and trees tower over them, or if they are growing in a crowd of other plants. It’s much like humans seeking out sunny spots or reaching their hands toward a campfire to warm up when they feel cold outdoors.
Humans twist or bend by changing our body positions temporarily, but when plants twist, bend or elongate, they are actually growing toward the light. The types of plants that reposition themselves to see the light are species that grow in a slow but determined way.
Other types of plants may not grow straight because they have different strategies. For example, strawberries grow close to the ground and spread sideways by sending out runners – stems that spread out just above the ground to create new plants.
Other plants, like ivy, grow as vines that climb up trees, walls and fences and use them for support. Climbing vines may grow straight, to the side or at angles, depending on what kinds of support structures they find to grow on. The purpose is to expose their leaves to as much sunlight as possible.
In my recent book, “Lessons from Plants,” I explore how plants are usually positioning themselves to see the light. It’s fascinating that we humans, too, are often positioning ourselves to see something interesting.
So the next time you see a plant growing straight, take notice of whether light is directly above it. Or if you see a plant that’s not straight, notice whether it’s bending toward light coming from the direction it’s facing. Or maybe it’s a vine climbing on a structure and using that support to take a different route toward the sun.
Hello, curious kids! Do you have a question you’d like an expert to answer? Ask an adult to send your question to CuriousKidsUS@theconversation.com. Please tell us your name, age and the city where you live.
And since curiosity has no age limit – adults, let us know what you’re wondering, too. We won’t be able to answer every question, but we will do our best.
–Beronda L. Montgomery
Professor of Biochemistry and Molecular Biology & Microbiology and Molecular Genetics,
and Assistant Vice President of Research & Innovation,
Michigan State University