Plant Tropisms
Investigating Plant Movements Generated by Growth
Plants move in response to both internal and external signals known as stimuli. Plant movements fall into two categories: tropisms and nastic movements.
Tropisms are responses in which the direction of the movement is determined by the direction of the stimuli. Tropisms are growth movements that happen slowly and whose results are irreversible.
If a plant reacts toward the stimuli, this is said to be a positive tropism but if the plant reacts away from the stimuli, this is termed a negative tropism. Plants respond in this fashion to the external forces of gravity, contact (touch), direction of light, water, and fluctuation in temperature.
Geotropism is Growth Influenced by Gravity
The effect of gravity upon the direction of growth is termed geotropism (or gravitropism) In plants, two main response exist to gravity: negative geotropism, as in the upward growth of the shoot of a plant, and positive geotropism, as in the downward growth of the root of a plant.
Growth of plant parts towards or away from gravity is controlled by phytohormones, primarily auxin. Higher or lower concentrations of auxins in various places in roots and stems stimulate growth toward or away from gravity.
Thigmotropism is Growth Influenced by Contact
Thigmotropism is the term applied to growth movements made by plants in response to contact with a solid object. Most such movements are curvatures and they are most impressive in the tendrils of climbing plants. A tendril is a thread – like growth that may be a modified leaf or part of a leaf, or a modified branch. Certain plant families, such as the pea, grape, and gourd, are outstanding in their climbing habits.
The curvature of the tendril that follow contact with a support are the result of increased growth on the side opposite the stimulus. Contact with a solid object causes the auxin to migrate to the side away from the contact. The cells on the far side of the contact lengthen more than those on the contact side and thus the tendril grows around the contact object. In the gourd family (pumpkin, squash, cucumber) a complete coil may form around a support within 1 to 10 minutes after contact, depending on the age of the tendril.
Phototropism is Growth Influenced by Light Direction
Phototropism is usually a positive response to light coming in greater intensity from one direction than another. Stems and leaves curve or bend (grow) toward the source of light. Roots, on the other hand, are negative (grow away) or are neutral to light direction. Stems bend directly toward the light but leaves become oriented in such a manner that the leaf blade is approximately at right angles to the light source. Thus, no leaf shades another.
Phototropism is a growth movement and the driving force for it is, as with geotropism, auxin. Light causes the hormone to migrate to the shaded side of the stem. The increased concentration of auxin on the shaded side causes the cells there to elongate more than those on the light side. As a result, the shoot bends toward the light exhibiting positive phototropism.
Vernalization is a Temperature Response
Vernalization is a response in which low temperatures stimulate flower formation. Vernalization is important for fall – sown crops, such as winter wheat, barley, and rye. For example, wheat seeds are planted in the fall and survive the winter as small seedlings. Exposure to cold winter temperatures cause the plants to flower early in the spring, and an early crop is produced. Cold temperatures are not absolutely required for most cultivars, but they do hasten flowering. Farmers can use this to their advantage to grow and harvest certain crops before summer heat and possibly drought sets in.
The coiling of tendrils, the bending of plants toward light, and the downward growth of roots appear to be the product of thoughtful behavior and while it may be popular to see it that way, this is false. One must be careful, especially with plants, not to attribute human needs and behaviors to them. Plants are merely responding to changing conditions inside and outside themselves within the parameters set by their genes.
Tropisms Regulate Plant Growth
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