Tropisms
Tropisms
Tom Ogren
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.
Electro-tropisms
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 (Hyrdrotropism)
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.
Photo-tropism
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, like some cultured gardeners, do indeed
turn toward good music. If so, we could call this what?
Music-tropism?
Thomas Leo Ogren is the author of Allergy-free Gardening