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Plants are thought by some as less human, less important than animals, because plants are not "salient" beings. They just sit there, they don't move. But actually they do. A tropism is an organism's response, or movement to (or away from) some external stimulus. Usually this refers to plants and to a plant's growth toward some sort of stimulation. Perhaps the most obvious tropism is that of the sunflower, which turns its "face" toward the sun, and actually follows the sun. If you want to photograph a glorious field of actively growing and blooming sunflowers, in the morning the flowers will be facing the eastern sun but in the evening they will be oriented toward the setting sun in the west.

This sunflower tropism is an example of phototropism, but more on that in a moment.

"Trop" comes from the Greek and means "turning toward." The ending, "ism," is also Greek, and means "a system." Thus a tropism is a system for plants to turn toward something.


Because of the work I do with pollen and allergies, I am especially fascinated by tropisms, in particular electro-tropism. A large, mature female red maple tree, for example, will have many thousands of tiny red female flowers. These flowers look like minute versions of the double winged seeds they will eventually evolve into. The tip of each female organ, the stigma, will be sticky and it will also produce a small negative electrical current. Because the tree's roots are sunk into the ground, the tree itself is grounded, thus negative.

Pollen from the male flowers on the male red maples trees will be released with the wind. Most of this pollen will fall, land, and stick close to the dripline of the tree, but some individual grains, each one shaped like a tiny football, will catch the breeze and float on the air. As the airborne pollen grains tumble about, they pick up a positive electrical current.

Because the female flowers are negative, and the pollen is positive, the two are mutually attractive. The airborne pollen grains will attract to and stick on the receptive female flowers. This is an example of an electro-tropism at work. The end result is seed.

A pollen researcher I know recently had an interesting experience with electro-tropism. She had just cleaned off the lens of her microscope with a cleaning solvent and was viewing some fresh alder pollen. The microscope lens must have still been moist and as she looked through the scope pollen grains started to jump up from the slide and stick to the lens. The grains did not jump one after another, but rather a few seconds would pass and then suddenly an entire clump of grains would spring from the slide. After a few moments another bunch of pollen would again suddenly rise from the slide and stick to the lens.

Chemo-tropisms and Aqua-tropisms (Hydrotropism)

The roots of a plant will actually reverse their path and will grow in the direction of soil nutrients. Most typically this will be root growth towards sources of available nitrogen, phosphorus or potassium, the macronutrients. Sometimes though roots will re-direct themselves towards sources of soil iron or sulfur or toward micronutrient soil chemicals such as zinc, magnesium, or selenium. In row cropping where the irrigation is always done on one side of the row only, and where water-soluble fertilizers are used along with the irrigation, roots will grow directly to this fertilizer.

Roots will also grow towards sources of water, as anyone with a willow tree near a septic line usually knows only too well. This is an example of aqua-tropism, which is also sometimes called hydrotropism. Gardeners, who soak the roots of a tree on only one side, will end up with a tree that has most of its roots only on one side. A tree like this would be more vulnerable to blowing over in a hard wind.

Geo-tropism or Gravitropism

Even though gravity affects everything on earth, most plants are geo-tropic and their roots will grow down and their tops will grow up. This is often demonstrated by placing a containerized plant on its side on the ground. If the plant is watered and otherwise properly taken care of, eventually the roots in the pot will change directions, and will start to grow down, even though "down" is now the side of the pot, not the bottom. This is most easily demonstrated by using an actively growing herbaceous plant like a tomato plant.

Likewise, the top of the plant will start to bend, to change directions, and eventually it will lift itself so that the top is now growing upward. If later this same plant is taken out of its container and planted in the ground, it will again exhibit geo-tropic tendencies, and will once more re-direct its roots downward and its top toward the sky.

Another good example of geotropism, suggested to me by Grandiflora editor Donna Williamson, is that of bulbs. If a plump daffodil bulb is planted incorrectly, upside down, it will produce a shoot that will emerge from the bulb heading the wrong direction, but that will then bend, change directions, and will eventually come out of the ground as it should. Likewise the roots of the up-side-down bulb will start to grow and will quickly reverse directions, and will grow down as daffodil roots were intended to do so. But if a bulb is small, weak, or planted too deep, and it is inverted like this, it may never emerge. It may run out of energy and die before the top reaches the sunlight.

Sometimes in a nursery workers will accidentally stick cuttings of dormant wood upside down, or inverted. With some species an inverted cutting will never root and will just die. But with a few species the inverted cutting will root, the roots will grow down, and the lateral buds that sprout, will first go down but will quickly change direction and grow upward.


One of the first people to understand this process was Charles Darwin who did experiments on this response. Photo is Greek for "light," and a phototropic response is one directed at growing toward the light, often toward the sun. Interesting things are at work to make a plant phototropic. Take the sunflower again-in the main stem of the sunflower plant is a naturally occurring growth hormone, called indole acetic acid. At night the indole acetic acid, which is light shy, will be spread though out the entire plant. But with the morning light, coming from the eastern sky, the indole acetic acid will shy away from the direct light shining on the eastern side of the plant, and will concentrate on the western side of the stem. Here the growth hormone will push extra growth of the tissues on the western side and this will cause the head of the sunflower to bend into the eastern sun.

By mid day the indole acetic acid will be focused directly under the large flower head, shaded from the direct sunlight. The hormone will then cause stem elongation and will further push the sunflower head toward the overhead sun.

As the afternoon progresses and the sun shifts to the western sky, the light-shy indole acetic acid gravitates now to the eastern side of the sunflower trunk and here it promotes growth of the tissues on that side. As the eastern side of the trunk grows larger and taller, the flower head is turned so that eventually it is turned to catch the setting western sunlight.

When we see a bright patch of iceplant flowers that open wide at mid-day and close up late in the day, or on cloudy days, this too is an example of phototropism at work.

Touch-tropism or Haptotropism or Thigmotropism

Response to touch or contact is a tropism. This can be seen in the reactions of the carnivorous plants. The sticky hairs of a Sundew plant will wrap around an insect when it comes into contact. Anyone who has ever had a potted Mimosa pudica, a Sensitive Plant, has seen this tropism at work. A little touch of a finger on a leaf and suddenly the entire leaf closes up. I used to always grow a few of these each year when I taught horticulture in the prison. My students would touch these Mimosa plants so often that sometimes they would die from what I assumed was over-stimulation. We sometimes see a dwarf, stocky tree growing in an open and over-exposed spot. There are some much-photographed Monterey Cypress trees, hanging precariously on the edge of steep cliffs above the Pacific Ocean. The trunks of these trees have grown thick and short from the constant stimulation of the ever-blowing wind. In a different but similar observation, on a recent trip through Maryland I drove along a country road and in several places I saw contorted and twisted trees growing directly underneath low-lying power lines. I would assume that this was a result of the trees' reactions to all that stray voltage.

Thermotropism, also called Thermonasty

Some plants do respond and move when stimulated by temperature changes. Petals of crocus and tulip will open at temperatures of around sixty degrees Fahrenheit and will close below this. Some green algae have been observed that swim toward areas of optimum temperature.

Other tropisms?

There may well be other as yet undiscovered tropisms at work in the kingdom of plants. When I owned a dairy farm my cows used to produce more milk to classical music. I used to play Mozart and Chopin in my nursery greenhouses as well, and just assumed it was beneficial for my plants. I have no proof of this, but perhaps some plants do indeed turn toward good music. If so we could call this what? Music-tropism?

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