Directional Response of Plants to the Touch of an Object: Thigmotropism

There are some plants called ‘climbing plants’ which have weak stems and hence cannot stand upright (or erect) on their own. The climbing plants have climbing organs called tendrils. Tendrils are the thin thread-like growths on the stems or leaves of climbing plants. Thus, there are two types of tendrils: stem tendrils and leaf tendrils. Tendrils are sensitive to the touch (or contact) of other objects.

That is, tendril has cells which can sense their contact with a nearby solid object like a bamboo stick, or the stem of another plant. So, when a tendril touches an object, then the sides of tendril in contact with the object grows slowly than its other side. This causes the tendril to bend towards the object by growing towards it, wind around the object and cling to it (see Figure 15). The winding movement of the tendril of a climbing plant is an example of thigmotropism. The stimulus in thigmotropism is the touch (or contact) of an object. The winding movement of the tendril of a plant around a nearby object gives support to the plant having a weak stem.

Thigmotropism is often seen in plants having tendrils. Tendrils are positively thigmotropic which means that they grow towards things they happen to touch. The plants having stem tendrils or leaf tendril which are positively thigmotropic climb up artificial supports, other plants or fences very easily. The plan 1 such as bitter gourd (karela), bottle gourd (lauki), grape vine and passion flower have stem tendrils which are positively thigmotropic and make these plants to climb up by winding around various types of supports. The plants such as peas and glory lily have leaf tendrils which are positively thigmotropic.

These leaf tendrils also make their plants to climb up by winding around various types of nearby supports. From the above discussion we conclude that tendrils are the climbing organs of the plants which are positively thigmotropic.

The Usefulness of Tropic Movements :

The various types of tropic movements help the plants to survive. For example, even if a seed is planted upside down, its root will still grow downwards into earth because it is positively geotropic. The root will also grow towards water because it is positively hydrotropic. Similarly, the shoot of such a seed will grow upwards because it is negatively geotropic and towards light because it is positively phototropic. These tropic movements help the plants to obtain water and nutrients from soil and light from the sun, which are necessary for their growth and survival.

Nasties (or Nastic Movements) :

We have just studied that in tropism, a plant part either moves towards the stimulus or away from the stimulus. However, in some plants, the movement of the plant part is neither towards the stimulus nor away from the stimulus. That is, the movement of plant part in some plants is not in a particular direction with respect to stimulus.

The movement of a plant part in response to an external stimulus in which the direction of response is not determined by the direction of stimulus is called nastic movement. Nastic movements of plants are also called nasties. The nastic movements of plants are induced by stimuli such as heat, light, touch (or contact), etc.

The main difference between tropic and nastic movements is that tropic movement is a directional movement of a plant part but nastic movement is not a directional movement of the plant part with respect to the stimulus. The direction of nastic movement is not determined by the direction from which the stimulus is applied. In nastic movement, from whichever direction the stimulus is applied, it affects all the parts of the organ of a plant equally and they always move in the same direction. Nastic movements are mostly exhibited by the flat organs of the plants like ‘leaves’ and ‘petals of flowers’. Some of the examples of the nastic movements of plants (or nasties) are given below:

(i) The folding up of the leaves of a sensitive plant (Mimosa pudica) on touching is an example of nastic movement. Here the stimulus is touch.

(ii) The opening up of the petals of dandelion flowers in morning in bright light and closing in the evening when the light fades is an example of nastic movement. In this case the stimulus is light.

(iii)The closing of the petals of moonflower in the morning in bright light and opening at dark when the light fades is also an example of nastic movement. In this case also the stimulus is light.

Please note that though all tropisms are growth movements but all nasties (or nastic movements) are not growth movements. Nastic movements may or may not be growth movements. For example, the folding up of the leaves of a sensitive plant on touching is not a growth movement but the opening and closing of petals of flowers by the action of sunlight is a growth movement.

We have just said that most of the movements of the plant parts are caused by their growth. Now, since the growth of a plant part is usually a slow process, therefore, most of the movements of plant parts are very slow. There are, however, some exceptions. We will now describe the movement of a plant part (leaves) which is unusually fast and takes place almost immediately. It is the folding up of the leaves of a sensitive plant when touched with a finger (or any other object). This is discussed below under the topic on thigmonasty.

Thigmonasty:

The non-directional movement of a plant part in response to the touch of an object is called thigmonasty. In other words, thigmonasty is the nastic movement of a plant part in response to touch. Thus, the stimulus in thigmonasty is the ‘touch’. An example of the nastic movement in plants caused by touch (or thigmonasty) is provided by the sensitive plant (Mimosa pudica) which is also known as touch me-not plant. It is called chhui-mui in Hindi. If we touch the leaves (or rather leaflets) of the sensitive plant with our fingers, then its leaves fold up and droop almost immediately. The folding up of the leaves of sensitive plant on touching is an example of nastic movements in plants (in which the stimulus is the ‘touch’ of our fingers).

The sensitive plant has pad-like swellings called ‘pulvini’ at the base of each leaf. (The singular of pulvini is pulvinus). The pulvini contain a lot of water in their cells. Due to the internal ‘water pressure’ in them (called turgor), all the pulvini are very firm and hold the leaves above the upright.

The pulvini have also large intercellular spaces (empty spaces) between their cells. The folding up of the leaves of a sensitive plant on touching is due to the sudden loss of water from pad-like swellings called ‘pulvini’ present at the base of all leaves of the sensitive plant which make the pulvini lose their firmness causing the leaves to droop and fall. This happens as follows.

When the leaves of sensitive plant (having pulvini at their base) are touched with a finger, then an electrical impulse is generated which travels through ordinary cells (because there are no nerve cells in sensitive plant or other plants).

This electrical impulse acts on a plant hormone. The plant hormone makes the water migrate from the cells of one half of a pulvinus to the intercellular spaces in the other half of pulvinus. This loss of water from half of pulvinus causes the pulvinus to lose its firmness making the leaf to fold. Similarly, all the pulvini lose firmness and become limp due to which all the leaves above them collapse and fold up.

At a gap of 15 to 30 minutes after the leaves have folded, water usually diffuses back into same cells of pulvinus from which it left, and the leaf returns to its original position.

Photonasty :

The non-directional movement of a plant part (usually petals of flowers) in response to light is called photonasty. In other words, photonasty is the nastic movement of a plant part (like petals of flowers) in response to light. Thus, the stimulus in photonasty is light, A dandelion flower opens up in the morning in bright light but closes in the evening when the light fades and it gets dark.

The opening and closing of petals of dandelion flowers in response to the intensity of light is an example of nastic movement in which the stimulus is light. In other words, it is an example of photonasty. The moonflower behaves exactly opposite to that of dandelion flowers in respect of response to light. The petals of moonflower close during the day when there is bright light but open up at night when it is dark and there is no light. This is also an example of photonasty.

Please note that the opening and closing of flowers in response to light (or photonasty) are growth movements. Petals open when their inner surfaces grow more than their outer surfaces. On the other hand, petals close when their outer surfaces grow more than their inner surfaces. Before we end this discussion, we would like to give the functions of plant hormones.