Download Plant Responses

Middle School Science
Room 212 – Miss Lida
Lesson 2 – Explore – Page 279
“Plant Responses”
Stimuli and Plant Responses
 Stimuli are any changes in an organism’s environment that cause a response.
 Often a plant’s response to stimuli might be so slow that it is hard to see it happen. The
response might occur gradually over a period of hours or days.
 Light is a stimulus. A plant responds to light by growing toward it. This response occurs
over several hours.
 In some cases, the response to a stimulus is quick, such as the Venus Fly trap’s response
to touch. When stimulated by an insect’s touch, the two sides of the trap snap shut
immediately, trapping the insect inside.
Environmental Stimuli
 Plants respond to their environments in a variety of ways. M<any trees flower in the
spring and drop their leaves in the fall. Both are plant responses to environmental
stimuli.
Growth Responses
 Plants respond to a number of different environmental stimuli. These include light,
touch, and gravity.
 A tropism is a response that results in plant growth toward or away from a stimulus.
 When the growth is toward a stimulus, the tropism is called positive. A plant bending
toward light is a positive tropism.
 Growth away from a stimulus is considered negative. A plant’s stem growing upward
against gravity is a negative tropism.
Light
 The growth of a plant toward or away from light is a tropism called phototropism.
 A plant has a light-sensing chemical that helps it detect light. Leaves and stems tend to
grow in the direction of light. This response maximizes the amount of light the plant’s
leaves receive.
 Roots generally grow away from light. This usually means that the roots grow down into
the soil and help anchor the plant.
Touch
 The response of a plant to touch is called thigmotropism.
 A tendril wrapping around an object is an example of positive thigmotropism.
 Roots display negative thigmotropism. They grow away from objects in soil enabling
them to follow the easiest path through the soil.
Gravity
 The response of a plant to gravity is called gravitropism. Stems grow away from gravity,
while roots grow toward gravity.
 No matter how a seed lands on soil, when it starts to grow, its roots grow down into the
soil. The stem grows up. This happens even when a seed is grown in a dark chamber,
indicating that these responses can occur independently of light.
Flowering Responses
 Photoperiodism is a plant’s response to the number of hours of darkness in its
environment.
 Scientists once hypothesized that photoperiodism was a response to light. Therefore,
these flowering responses are called long-day, short-day, and day-neutral, and relate to
the number of hours of daylight in a plant’s environment.
Long-Day Plants
 Plants that flower when exposed to less than 10 – 12 hours of darkness are called longday plants.
Short-Day Plants
 Short-day plants require 12 or more hours of darkness for flowering to begin. (An
example of a short-day plant is the poinsettia).
Day-Neutral Plants
 The flowering of some plants doesn’t seem to be affected by the number of hours of
darkness. Day-neutral plants flower when they reach maturity and the environmental
conditions are right. Plants such as roses are day-neutral plants.
Chemical Stimuli
 Plants respond to chemical stimuli as well as environmental stimuli.
 Plant hormones are substances that act as chemical messengers within plants.
 These chemicals are produced in tiny amounts.
 They are called messengers because they usually are produced in one part of a plant
and affect another part of that plant.
Auxins
 One of the first plant hormones discovered was auxin.
 There are many different kinds of auxins.
 Auxins generally cause increased plant growth.
 They are responsible for phototropism, the growth of a plant toward light.
 Auxins concentrate on the dark side of a plant’s stem, and these cells grow longer. This
causes the stem of the plant to grow toward the light.
Ethylene
 The plant hormone ethylene helps stimulate the ripening of fruit.
 Ethylene is a gas that can be produced by fruits, seeds, flowers, and leaves.


Rotting fruits release ethylene.
Ethylene can also cause plants to drop their leaves.
Gibberellins and Cytokinins
 Rapidly growing areas of a plant, such as roots and stems, produce gibberellins. These
hormones increase the rate of cell division and cell elongation. This results in increased
growth of stems and leaves.
 Root tips produce most of the cytokinins, another type of hormone.
 Xylem carries cytokinins to other parts of a plant.
 Cytokinins increase the rate of cell division, and in some plants, cytokinins slow the
aging process of flowers and fruits.
Summary of Plant Hormones
 Plants produce many different hormones.
 Often, two or more hormones interact and produce a plant response.
 Scientists are still discovering new information about plant hormones.
Humans and Plant Responses
 Humans depend on plants for food, fuel, shelter, and clothing. Humans make plants
more productive using plant hormones.
 Some plants are now easier to grow because humans understand how they respond to
hormones.