Moments of Curvature
Since the higher plants are fixed to the soil, they cannot move from one place to another. Certain organs of these plants, show spontaneous movement forming curvature the movements may or may not be accompanied by growth. If the curvature is permanent and accompanied by growth, it is called the movement of growth. If the curvature is temporary and not accompanied by growth, it is known as the movement of variation.
If the movement of curvature exhibited by plant organs is due to some external stimuli (caused by light, temperature, gravity, foreign bodies etc.), it is called induced movement of curvature, l. Spontaneous movements of curvature
(i) Nastic. Movements – It occurs in leaves, flowers, petals, bud scales etc. In such organs of the plant, growth may become unequal, may be on the upper surface or lower surface. If the growth is more in the upper surface, the movement is called epinasty. If the growth is more on the lower surface, the movement is hyponasty.
The upward bending of a prostrate or a drooping structure, the folding in of a folding in of a plant part arc the examples of hyponastic movements
(ii) Nutation – It is a spontaneous movement in curvature due to unequal growth of plant organs. Stems of many plants move in zig zag fashion due to more growth on one side followed by the other. In many climbers, stem follows a spiral path in space. This is called circumnutation. It happens due to spiral shift around the axis of the regions of active growth.
Paratonic Growth Movements of Curvature
In many cases, growth movements are induced by certain external stimuli. The stimuli can cause growth movements when they are unilateral. Such movements are called tropic movements and the phenomenon is known as tropism.
Depending on the nature of the external stimuli like light, gravity, water, touch and chemicals, the movements are called phototropism, geotropism, hydrotropism, thigmotropism and chemotropism, respectively.
When a plant organ curves due to unilateral light stimulus, it is called phototropism. Some parts of the plants such as the stems move towards the light.
These organs are positively phototropic. Some other organs like root move away from light and they are called negatively phototropic. This can be shown in a phototropic chamber
Growth movements induced by stimulus of gravity are known as geotropism. Growth of curvature towards the gravitational force is known as positive geotropism (roots) and growth away from the gravitational force is negative geotropism (stem). When an organ grows at right angles to the direction of gravity, it is called diageotropic (rhizomes, stolons).
When an organ moves (curves) o to 90° or 90° – 180° from vertical line, it is called plageotropism (leaves, lateral branches, and secondary roots). Some organs do not respond to geotropic stimulus, and are called ageotropic (example – coralloid roots of Cycas).
It is a movement induced by source of water. Roots are positively hydro’tropic.
It is a movement induced by contact with a solid object. It is seen in the tendrils of climbing plants like cucumbers, pumpkins etc.
Fungal hyphae and pollen tubes exhibit positive tropic movement under the influence of certain chemical substances. The chemicals are normally sugars and other nutritional substances.