Environmental Engineering Reference
In-Depth Information
t
ropisms
Tropism is the movement (including growth in plants) of an organism in response to
an external stimulus. Tropisms controlled by hormones are a unique characteristic
of sessile organisms such as plants that enable them to adapt to different features of
their environment—gravity, light, water, and touch—so they can flourish. The three
main types of tropisms are
•
Phototropism
—The tendency of plants to grow or bend (move) in response
to light. Phototropism results from the rapid elongation of cells on the dark
side of the plant, which causes the plant to bend in the opposite direction. For
example, the stems and leaves of a geranium plant growing on a windowsill
always turn toward the light.
•
Gravitropism
—The tendency of plants to grow toward or against gravity. A
plant that displays positive gravitropism (plant roots) will grow downward,
toward the center of earth. That is, gravity causes the roots of plants to grow
down so that the plant is anchored in the ground and has enough water to
grow and thrive. Plants that display negative gravitropism (plant stems) will
grow upward, away from the earth. Most plants are negatively gravitropic.
Gravitropism is also controlled by auxin. In a horizontal root or stem, auxin
is concentrated in the lower half, pulled by gravity. In a positively gravit-
ropic plant, this auxin concentration will inhibit cell growth on the lower
side, causing the stem to bend downward. In a negatively gravitropic plant,
this auxin concentration will inspire cell growth on that lower side, causing
the stem to bend upward.
•
Thigmotropism
—The tendency of plants to grow or bend in response to
touch. Some people notice that their houseplants respond to thigmotro-
pism by growing better when they are touched and have attention paid to
them. Touch causes parts of the plant to thicken or coil as they touch or are
touched by environmental entities. For example, tree trunks grow thicker
when exposed to strong winds and vines tend to grow straight until they
encounter a substrate to wrap around.
p
hotoperiodism
Photoperiodism is the response of an organism, such as plants, to naturally occurring
changes in light during a 24-hour period. The site of perception of photoperiod in
plants is leaves. Sunflowers are particularly well known for their photoperiodism, or
their ability to open and close in response to the changing position of the sun through-
out the day. All flowering plants have been placed in one of three categories with
respect to photoperiodism:
•
Short-day plants
—Flowering is promoted by day lengths shorter than a
certain critical day length; includes poinsettias, chrysanthemums, golden-
rod, and asters.
