A growth movement of a plant part in response to an external stimulus in which the direction of stimulus determines the direction of response is called tropism. Thus, tropism is a directional movement of the part of a plant caused by its growth. The growth of a plant part in response to a stimulus can be towards the stimulus (in the direction of stimulus) or away from the stimulus (against the direction of stimulus) due to which we can have a positive tropism or negative tropism, respectively. So:
1. If the growth (or movement) of a plant part is towards the stimulus, it is called positive tropism, and
2.If the growth (or movement) of a plant part is away from the stimulus, then it is called negative tropism.
We will now give an example of tropism. When a growing plant is exposed to light from only one side, then it responds by bending its stem (or shoot) towards the light. This is an example of phototropism (which is caused by the ‘light’ acting as ‘stimulus’. ‘Photo’ stands for ‘light’). The bending of the plant stem (or shoot) towards light is actually positive phototropism.
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There are five common stimuli in the environment: light, gravity, chemicals, water and touch (or contact). These five stimuli give us five types of tropisms: phototropism, geotropism, chemotropism, hydrotropism and thigmotropism. In phototropism, the stimulus is light ; in geotropism the stimulus is gravity, in chemotropism the stimulus is a chemical, in hydrotropism the stimulus is water, and in thigmotropism the stimulus is touch (of a solid surface). It is obvious that the tropisms are named according to the stimulus. This will become clear from the following table.
Stimulus Type of tropism
Light Phototropism
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Gravity Geotropism
Chemical Chemotropism
Water Hydrotropism
Touch Thigmotropism
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We will now give the definitions of all the five types of tropisms.
(i) The movement of a plant part in response to light is called phototropism:
In other words, the response of a plant to light is called phototropism. If the plant part moves towards light, it is called positive phototropism. On the other hand, if the plant part moves away from light, then it is called negative phototropism. The stem (or shoot) of a growing plant bends towards light, so the stem (or shoot) of a plant shows positive phototropism (see Figure 5). On the other hand, the roots of a plant move away from light, so the roots of a plant show negative phototropism.
(ii) The movement of a plant part in response to gravity is called geotropism:
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In other words, the response of a plant to gravity is called geotropism. If the plant part moves in the direction of gravity, it is called positive geotropism. On the other hand, if the plant part moves against the direction of gravity, it is negative geotropism (Please note that the force of gravity acts in the downward direction). Now, the roots of a plant move downwards in the direction of gravity, so the roots of a plant show positive geotropism. On the other hand, the stem (or shoot) of a plant moves upwards against the direction of gravity, so the stem (or shoot) of a plant shows negative geotropism.
(iii) The movement of a plant part in response to a chemical stimulus is called chemotropism:
In other words, the response of a plant to chemical stimulus is called chemotropism. If the plant part shows movement (or growth) towards the chemical, it is called positive chemotropism. On the other hand, if the plant part shows movement (or growth) away from the chemical, then it is called negative chemotropism. The growth of pollen tube towards the ovule during the process of fertilisation in a flower is an example of chemotropism (It is actually positive chemotropism). In this case the pollen tube grows towards the sugary substance (chemical) secreted by the ripe stigma of carpel in the flower.
(iv) The movement of a plant part in response to water is called hydrotropism:
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In other words, the response of a plant part to water is called hydrotropism. If the plant part moves towards water, it is called positive hydrotropism. On the other hand, if the plant part moves away from water, then it is called negative hydrotropism. The roots of a plant always go towards water, so roots are positively hydrotropic.
(v) The directional growth movement of a plant part in response to the touch of an object is called thigmotropism:
The climbing parts of the plants such as tendrils grow towards any support which they happen to touch and wind around that support. So, tendrils of plants are positively thigmotropic.