Thigmotropism - Growth Movements of plants Organs - Haptotropism

The growth movements of plants organs in response to the stimulus of contact or friction are referred to as thigmotropism or haptotropism and are particularly seen in plants which climb by means of tendrils or in which ordinary parts serve for climbing because they are sensitive to the contact stimulus. The tendrils are found in a number of plants and are modified stem branches e.g., in the vine, Passiflora and Cucurbitaceous plants, leaves, leaflets or stipules.

In early stage the tip of the tendril notates in the air and if it comes in contact with some solid object it begins to coil around it. The side stimulated by contact becomes the inner side due to the fact that growth on that side is retarded while it remains normal or is accelerated on the opposite side. Only the apical part of the tendril is sensitive to contact. Soon after the initial coiling the stimulus of contact is transmitted backwards and basal part of the tendril also becomes spirally coiled, much like a coil spring but in reverse direction from the apical. This coil spring arrangement is of great advantage to the twiner; it draws the plant closer to the support and by alternate stretching and tightening prevents the breaking of the tendrils in strong wind.

In some plants the response to contact is very rapid, taking only a few seconds. It appear that, in such cases, at least the initial reaction is brought about by turgour changes on the opposite side of the stimulated tendrils and this is followed by a permanent growth reaction. It would thus appear that three different kinds of movements are involved in the twinning of tendrils. These are, in the sequences of their occurrence, autonomic nutations, turgour movements and permanent growth movements.

Thigmotropic responses are induced only when the tendrils are actively growing. Tendrils are sensitive to contact with only solid objects having rough surfaces. Liquids and solids with perfectly smooth surfaces fail to produce any reaction.

Perception of the contact stimulus is in some cases, as in certain species of Cucurbitaceae, associated with the presence of special “tactile pits” on the tendrils. These are unthickened areas in the outer walls of the epidermal cells so that protoplasm at these points is very near to the surface and is comparatively readily irritated by contact with solid bodies.

Thigmotropism responses are also met with in the roots. If the tip of a young root comes in contact with a solid object such as a small stone, the root bends away from it. This negative curvature helps the root to avoid obstacles in the soils. Thigmotropism also plays a part in the movements of the tentacles of Drosera due to mechanical stimuli.