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Plant Responses to the Environment. Pgs. 113 - 117. Plant Tropism. A stimulus is anything that causes a reaction. Plants respond to stimuli like gravity or light. Growth in a response to a stimulus is a tropism.
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Plant Responses to the Environment Pgs. 113 - 117
Plant Tropism A stimulus is anything that causes a reaction. Plants respond to stimuli like gravity or light. Growth in a response to a stimulus is a tropism. They can be positive or negative based on the plant growing toward or away from the stimulus. Two examples of tropisms are phototropism and gravitropism.
Sensing Light If a plant is placed so that it only gets light from one direction the stem will bend toward the light. This is an example of phototropism. There is growth of a plant caused by light. The cells away from the light grow longer and bend the stem toward the light.
Which way is up? Gravitropism is a plant changing direction of growth due to gravity. Most plants have negative gravitropismin which they grow up, away from the direction gravity is pulling them. Most root tips have positive gravitropism though because they grow straight down into the Earth.
Seasonal Responses A plant living in an area that has severe winters would have a hard time trying to flower in December. In fact, the plant would not be successful at all. Therefore, plants are able to detect change in seasons to help them survive.
Night and Day During fall and winter, the nights get longer and days shorter. The difference between night and day are stimuli for plants. The difference can cause plants to begin reproducing. Some plants flower early in the spring or late in the fall.
When the night is longer than day and a plant flowers, it is a short day plant. Examples of these are poinsettias, ragweed, and chrysanthemums. Long-day plants flower when the day is longer than night. Examples of these are clover, spinach, and lettuce.
Seasonal Changes in Leaves All trees lose their leaves at some time. Pine and holly shed some of their leaves year-round to keep leaves present all year. This is why they are called evergreens. Deciduous trees lose all their leaves at the same time each year. Bare branches in the winter help reduce water loss and survive low temperatures.
Leaves change color before fall because chlorophyll breaks down. As the green pigment is lost other colors show through. These yellow and orange pigments are always present but not shown when chlorophyll is present.
