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Varieties of Tropism
A tropism is considered by scientists to be a phenomenon of the biological variety in which a living organism turns or grows in response to stimulus from its environment. The direction the organism grows or turns in is dependent on the direction of the stimulus. The opposite of tropism is nastic movement, in which the response to stimulus is considered non-directional.
Tropisms are named for the stimulus which generates them. The word ‘tropism’ comes from the Greek word ‘trope’, which means ‘to turn’ or ‘to change’. Some of the varieties of tropism are chemotropism, geotropism, hydrotropism, heliotropism, phototropism, and thigmotropism.
Tropisms are usually associated with plants or other fixed organisms. If an organism is capable of physically moving by its own will or motility, its activity or movement in response to stimulus is not considered to be a tropism, but rather a taxis, which is a directional response, or a kinesis, which is a non-directional response.
Chemotropism occurs as a result of chemical stimulus, usually in plants or bacteria. A good example of this type of movement is evident during the growth of a pollen tube. It can be witnessed when lipids are present at the surface of the stigma, stimulating accelerated growth in the pollen tubes. These tubes can also be stimulated to grow even faster by the presence of more than one grain of pollen in the stigma of the flower.
Charles Darwin was the first to document the presence of geotropism, or gravitropism, which is the turning or growth movement of a plant or fungi in response to the Earth’s gravity. This is visible in the downward growth of the roots of plants and also in jungle vines which grow downwards from the tops of trees. It can also be seen by taking a close look at the growth directions of lichens and mosses on rocks.
Hydrotropism is the directional growth in response to the stimulus of water and its direction of movement. It is very difficult to observe in the roots of plants even though it is present since the action of water is not visible as it constantly courses through the soil and would require the disturbance of the subject in order to observe. But this process is easy to imagine, as the water thirsty roots of plants reach out, growing in the direction that will give them the best access to moisture.
Heliotropism is also referred to as the diurnal motion of plants. This term describes the plants’ movements in direct response to the movement of the sun across the sky. Flowers may assume a random orientation at night, but when the sun rises in the east, the flowers turn towards it and follow it across the sky as it sets in the west. This motion is accomplished by motor cells within the flexible segments of the stem, just below the flower. They do this by pumping potassium ions into their tissue, which changes the pressure, resulting in motion.
Phototropism is different from heliotropism in that it describes the motion of plants in response to light stimulus, but not just that of the sun. The growth of plants towards a light source is termed positive phototropism while growth away from a light source is termed negative phototropism. Most plants experience positive phototropism while their roots exhibit negative phototropic tendencies as they grow deeper into the soil. Many mosses and lichens are phototropic and can be found to grow on the parts of rocks that are exposed to the sun while mold and mildew grows in the areas that receive no sunlight.
Some climbing plants such as vines exhibit thigmotropic reactions to the stimulus of touch or contact. Plants that react in this way contain cells that produce auxin, which causes them to move as they grow around surfaces such as walls, pots, or poles.
1. The word ‘associated’ in the passage is closest in meaning to _____________
| A. compared | B. separated | C. linked | D. handled |
| 2. . According to paragraph 2, tropisms are named for _____________ | |||
| A. the variety of affected plants | B. the stimulus that affects plants | ||
| C. the static movement of plants | D. the rate and direction of plant growth | ||
| 3. According to paragraph 3, which of the following is true about a taxis? | |||
| A. It is a kind of tropism. | B. It is a directed response to stimulus. | ||
| C. It is more responsive than a kinesis. | D. It impairs motility. |
4. Which of the following can be inferred from paragraph 4 about chemotropism?
A. Adding pollen to a flower creates a chemical stimulus.
B. The action of water is always visible in plants.
C. Lipids are a part of a plant.
D. Pollen is a natural inhibitor of plant growth.
5. According to paragraph 5, which of the following is a sign of geotropism?
A. vines climbing a tree trunk
B. fungi growing on another plant
C. the downward lichen growth of a rock
D. roots appearing above round
6. The word ‘it’ in the passage refers to _____________
A. heliotropism
B. stimulus of water
C. direction of movement
D. action of water
7.. According to the last paragraph, which of the following is NOT typical of climbing vines?
A. They exhibit thigmotropism reactions.
B. They contain auxin.
C. They are affected by touch.
D. They harbor mold and mildew.
8. Which of the following is TRUE about Phototropism?
A. It describes the motion of plants in response to the sun.
B. Its negative category affects the downward growth of roots.
C. It stimulates growth in the pollen tubes with lipids.
D. It explains the growth movement of a fungi in response to the Earth’s gravity.
9. Which of the following is TRUE about Heliotropism?
A. It is present in plants that pump potassium ions into their tissue.
B. It explains why some plants grow around surfaces like walls and poles.
C. It causes the roots of plants to grow downward from tops of trees.
D. It is caused by chemical stimulus normally in plants or bacteria.
10. Which of the following is TRUE about Hydrotropism?
A. It causes moss to grow on rocks exposed to sunlight.
B. It demonstrates the movement of plants in response to the sun.
C. Its positive category stimulates the growth of plants.
D. It involves a directional response to water.
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