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(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.
The growth (or movement) of a pollen tube towards the ovule induced by a sugary substance as stimulus, is an example of chemotropism.
The ripe stigma in the carpel of a flower secretes a chemical substance (which is a sugary substance) into the style towards the ovary
This sugary substance acts as a stimulus for the pollen grains which fall on the stigma of the carpel.
The pollen grain responds to this stimulus by growing a pollen tube in the downward direction into the style of the carpel and reaches the ovule in the ovary of the flower for carrying out fertilisation.
This growth of the pollen tube in response to a chemical substance secreted by the stigma of a flower is an example of chemotropism.
(iv) The movement of a plant part in response to water is called hydrotropism.
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
The roots of plants always go towards water, even if it means going against the pull of gravity.
Though roots normally grow downwards but in order to reach water, they can grow sideways or even upwards !
The roots grow in the direction of source of water so as to obtain water for the developing plant.
Since roots always grow (or move) towards water, therefore, roots are positively hydrotropic.
In trough B we plant a similar seedling and also place a small ‘clay pot’ inside the soil.
Do not water the soil in trough B but put some water in the clay pot buried in the soil
Leave both the troughs for the root of seedling in trough B is found to be bent to the right side (towards the clay pot containing water)
(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
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, tendrils have 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 side 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 .
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.
The plants 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 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.