Looking out of my kitchen window recently, I noticed a foxglove (Digitalis purpurea) with a tall bud spike. Watching it develop, I noticed that during the day it moved with a rather extreme bending action. I dug out my botany books and was reminded that plants can move in a variety of ways. While we normally think of plants as being rooted to a specific spot, they seem sneakily ingenious in their ability to move around!
Plants move in response to light. I think that is what my foxglove was doing, moving towards first the morning and then the afternoon sunlight. This movement is called phototropism. Specialized hormone cells, known as auxins, control growth by stimulating cell elongation. It is well accepted that phototropic bending of stems and roots results from cells on one side elongating faster than cells on the other side. This causes the plant to bend and direct its growth either toward available sunlight (positive phototropism) or away from it (negative phototropism).
Plants also move in response to touch or external stimulus. The mimosa tree (Albizia julibrissin) and oxalis houseplant (Oxalis regnelliiandOxalis triangularis) both fold their leaves when touched or disturbed. This type of movement is called nastic (from the Greek word meaning to press). Perhaps the most familiar response to touch is the Venus flytrap (Dionaea muscipula) snapping shut when an insect touches two of the flytrap's sensitive hair triggers. Other more benign movement occurs in some flowers, such as when the moss rose (Portulaca grandiflora) closes its petals at night.
Anyone who has seen a morning glory coiling around a support has observed thigmotropism in action. This response occurs following a "force contact," in which the direction of a tendril curves toward the rigid surface that it contacted. The tendril then coils around that surface. At the cellular level, a combination of cell elongation and changes in cell pressure are responsible for generating growth along or around a solid object. Some tendrils will begin to curve within less than a minute of a contact stimulus. Cell membrane protrusions transmit a signal that is rapidly acted upon by some unknown mechanism. Time-lapse photography has shown tendrils even waving around as though in search of a twining support.
Less familiar movements result from a plant's response to:
- temperature—thermotropism
- chemicals—chemotropism
- gravity—geotropism or gravitropism
- water—hydrotropism
Thermotropism, a plant's response to temperature, can be seen in winter when the leaves of rhododendrons (Rhododendron spp.) curl downward in extremely cold weather. This movement is thought to be a way of preventing water loss through stomata cells on the undersides of the leaves.
A plant cannot detect water at a distance. But if water is detected nearby, the plant uses hydrotropism to direct its root growth toward that water. Some tree species have a negative reputation because their roots find their way into water pipes and sewer systems, but they are simply taking maximum advantage of those nearby water sources. Plants also respond to water by rapidly growing when it is present and slowing growth down when it is not.
Roots exhibit a chemical response, chemotropism, by concentrating their growth in regions of higher nutrient concentration. Some are sensitive to chemical compounds that would signify the presence of beneficial and/or harmful bacteria, fungi, or even other plants.
Some plants move their seeds. The native woodland celandine poppy (Stylophorum diphyllum), the horrible weed hairy bittercress (Cardamine hirsuta), the native witch hazel tree (Hamamelis virginiana) and the annual vinca flower (Catharanthus roseus) are familiar examples of plants that spread via explosive seedpods. Just barely touch a mature seedpod and watch how it pops open, flinging seeds away from the parent plant.
Plants move and react to their environment in all kinds of ways and for all kinds of reasons. Think about this the next time you see a morning glory!
