Nyctinastic movements are plant movements in response to the daily cycle of light and dark. Nyctinastic movements involve the same type of osmotic mechanism as thigmonastic movements, but the changes in turgor pressure are more gradual. Nyctinastic movements occur in many plants. Examples of plants that demonstrate these movements include honeylocust trees, silk trees, and bean plants. The prayer plant gets its name from the fact that its leaf blades are vertical at night, resembling praying hands. During the day, however, the leaf blades are positioned horizontally. Carolus Linnaeus planted a “flower clock” made of different species of plants with nyctinastic movements. The movements of each plant species occurred at a specific time of day when the light was right for the plant.
Thigmonastic movements are a type of nastic movements that occur in response to touching or shaking a plant. Many thigmonasties involve rapid plant movements, such as the closing of the leaf trap of a Venus flytrap plant or the folding of a plant’s leaves in response to being touched. Some leaves of sensitive plants will fold within a few seconds after being touched. This movement is caused by the rapid loss of turgor pressure (water pressure) in certain cells, a process similar to that which occurs in guard cells in order to close stomata. Physical stimulation of the plant leaf causes potassium ions to be pumped out of the cells at the base of the leaflets and petioles. Water then moves out of the cells by osmosis. As the cells shrink, the plant leaves move. It is believed that the folding of a plant’s leaves in response to touch is to discourage insect feeding.
In addition, thigmonastic movements help prevent water loss in plants. When the wind blows across a plant, the rate of transpiration is increased. If the leaves of a plant fold in response to the “touch” of the wind, water loss is reduced.
Plant movements that occur in response to environmental stimuli, but that are independent of the direction of the stimuli are called nastic movements. These movements are regulated by changes in water pressure in certain plant cells.
Chemotropism is a plant growth response to a chemical. After a flower is pollinated, a pollen tube grows down through the stigma and style and enters the ovule through the micropyle. The growth of the pollen tube in response to chemicals produced by the ovule is an excellent example of chemotropism.
Gravitropism is a plant growth response to gravity. A root usually grows downward and a stem usually grows upward; that is, roots are positively gravitropic and stems are negatively gravitropic. Like phototropism, gravitropism appears to be regulated by auxins. One hypothesis proposes that when a seedling is placed horizontally, auxins accumulate along the lower sides of the root and the stem. This concentration of auxins stimulates cell elongation along the lower side of the stem, and the stem grows upward. A similar concentration of auxins inhibits cell elongation in the lower side of the root, and thus the root grows downward.
Thigmotropism is a plant growth response to touching a solid object. Tendrils and stems of vines, such as morning glories, coil when they touch an object. Thigmotropism allows some vines to climb other plants or objects, thus increasing its chance of intercepting light for photosynthesis. It is thought that auxin and ethylene are involved in this response.
Phototropism, as mentioned, is illustrated by the movement of sprouts in relation to a light source direction. Light causes the hormone auxin to move tot he shaded side of the shoot. The auxin causes the cells on the shaded side to elongate more than the cells on the illuminated side. As a result, the shoot bends toward the light and exhibits positive phototropism. In some plant stems, phototropism is not caused by auxin presence or movement. In these instances, light causes the production of a growth inhibitor on the illuminated side of the shoot. Negative phototropism is sometimes seen in vines that climb on flat walls where coiling tendrils have nothing to coil around. These vines have stem tips that grow away from the light or better put, toward the wall. This brings adventitious roots or adhesive discs in contact with the wall on which they can cling and climb.
Solar tracking is the motion of leaves or flowers as the follow the suns’ movement across the sky. By continuously facing toward a light source, moving or not, the plant maximizes the light available for photosynthesis.
A tropism is a plant movement that is determined by the direction of an environmental stimulus. The movement toward an environmental stimulus is called a positive tropism, and movement away from a stimulus is called a negative tropism. Each kind of tropism is named for its stimulus. For example, a plant movement in response to light coming from one particular direction is called phototropism. The shoot tips of a plant that grow toward the light source are positively phototropic.
Plants appear immobile because they are usually rooted in one place. However, time-lapse photography reveals that parts of plants frequently move. Most plants move too slowly for the passerby to notice. Plants move in response to several environmental stimuli, such as light, gravity, and mechanical disturbances. These movements fall into two groups: tropisms and nastic movements.