Stem senses
Touch
You might think that Plants are stationary organisms which are fixed to the ground. Or that the only movement they can make is when they are moved by the wind or passing animals. However, plants can move in surprising ways, and not all of them slow. Keen plant watchers since the 19th century have noticed patterns of plant movement and responses to touch. These responses can be immediate or happen gradually over time.
Plant responses to touch stimulation can be divided into two types:
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Thigmotropism: a directional response to touch towards the stimulus
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Thigmonasty: a non-directional response to touch which is unaffected by the direction of the stimulus
Key definitions
Stimulation: action which increases the levels of activity in the body or in biological systems.
Stimulus: any change in an environment that can be detected by an organism.
Thigmotropism: a directional response to touch towards the stimulus
Thigmonasty: a non-directional response to touch which is unaffected by the direction of the stimulus
Thigmomtropic responses
Climbing tendrils
Tendrils are modified leaves or stems which plants use to increase their vertical height, allowing them to increase their exposure to the sun. Tendrils use touch stimulation to anchor themselves to objects by coiling around them. The tip of the tendril is highly touch-sensitive, and when stimulated causes the plant tip to rapidly coil in the direction of the stimulus. However, climbing plants will not coil randomly and have been shown to be selective, coiling in a rainstorm for example would be pointless, as such tendrils wont coil when touched by water droplets.
Shy canopies
Plants use touch to sense their surrounding neighbours. Interaction between plants is normally competitive however an adaptation called crown shyness, shows plants reducing competition by leaving space between them and their neighbours. When blown by the wind plants will sway and normally brush up against other plants which are nearby. If a plant detects that there are other plants close by they will stop growth of their tissues in the direction of stimulus.
Windy woods
Mechanical stimulation caused by the wind has been shown to have a large effect on plant growth. Plants which grow in windier areas have been seen to be smaller, grow stronger and have larger trunks, to prevent damage by the wind.
thigmonastic responses
Carnivorous plants
Carnivorous plants utilise mechanical stimulation in many ways to trap and consume prey. One of the most famous examples is the Venus fly trap. This species reacts rapidly to physical touch by potential prey, by snapping closed its upper and lower leaves. This response is stimulated by the movement of tiny hairs on the inner surface of the leaves, these act as triggers telling the plant to close its leaves. Another carnivorous group of plants known was the sundew plants have sticky, colourful leaves. the sticky droplets sit on the end of modified leaves called tentacles. When stimulated by touch, the tentacles curl inwards trapping the unlucky victim.
The sensitive plant
The plant Mimosa pudica, or more commonly known as the sensitive/shy plant, has a unique response to physical touch. When mechanically stimulated the plant will quickly fold up its small leaves, stimulating the neighbouring leaflets on the same leaf to also fold up. The plant has also been observed to stimulate leaves further away to fold up, if one leaf is more seriously wounded. It is thought that this response is used to scare predators away, or make the plant less appealing to eat. Some close relatives of this plant species also react in this way. Other relatives have protective thorns on the underside of the leaf so that when they curl up they reveal their extra defences against predators.
Check out these amazing plants in action below
The Venus flap trap trapping its prey...
See how the Mimosa Pudica responds to touch ...
This video was kindly taken by Dr Maria Clara Castellanos of the University of Sussex
Summary
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Plants show both gradual and fast changes in movement in response to their environment
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Changes in movement can be directional (Thigmotropic) or non-directional (Thignmonastic)
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Directional changes are normally slower, and result in changes to the plants growth
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Non-directional responses are normally more immediate, and result in non-permanent changes to the plant