The thigmotropism it is a biological quality by which a plant moves when touched. It is a product of the ability to adapt and occurs in response to physical contact against a solid object, which obstructs the natural direction of growth..
This is a quality characteristic of plants, especially visible in those that bloom. However, some fungi are also capable of growing with thigmotropism..
The term has Greek roots and is a word composed of the prefix "tigmus", which means "to touch", added to the word "tropism", which means "turn.".
All plants are capable of developing thigmotropism, although not all do it to the same extent. It occurs slowly, but its appearance is irreversible, even if the stimulus-object is withdrawn.
Thanks to thigmotropism, a plant can adapt and grow on a trunk, wall or any object that gets in its way. To do this, they develop a special organ that allows them to adhere to the support.
In botany, this organ is called a “tendril”. There are tendrils of the foliar type, coming from the leaves; and there are tendrils of the caulinar type, coming from thin stems; so fine that they are not capable of producing flowers or leaves, but allow the plant to climb or crawl.
In the following video you can see the thigmotropism in the species Sundew burmannii:
There are two types of thigmotropism, positive and negative. They are not exclusive, so both can occur at the same time in the same plant.
It occurs when the plant grows around the object-stimulus, using it as a support to continue growing.
For it to occur, the presence of light is necessary, since the plant will grow looking for it. That is why this quality is present above all in flowering plants.
In experiments carried out by Mark Jaffe with bean seed sprouts, while a plant was in contact with an object and in turn exposed to light, the stem looked for a way to use the object as a support to reach the light, while when performing the same test in the dark, the stem remained straight.
The Vine (Vitis Vinifera), a plant from which the grape is born, has tendrils that, when touching a support, are wound around it. Once rolled, they harden to achieve more grip and thus grow satisfactorily.
Another clear example of positive thigmotropism is in climbing plants and vines.
They have achieved a higher level of adaptation, since they expand on the surface of the object and sometimes end up depriving other plants of CO2 and light..
It occurs only in the roots of plants, especially when the roots are still thin and weak; its function is to avoid the stimulus-object. The roots are incredibly vulnerable to touch, so they deviate at minimal resistance.
Negative thigmotropism is capable of canceling the roots' own geotropism. In experiments carried out by Charles Darwin with germinated bean seeds, he found that upon contact with an obstacle, the root deviates from its natural vertical growth..
Depending on the direction in which the plant or one of its parts rotates, it will be classified as dextrorotatory or left-handed thigmotropism.
Occurs when the turn is oriented to the right side, clockwise.
It is the opposite of dextrorotatory thigmotropism, it occurs when the plant turns to the left side, taking an anti-clockwise.
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