Irritability is the ability of a living organism to react to changes in its environment, or stimulus. A stimulus is a detectable change in the internal or external environment that triggers a response in an organism. A response is a change in an organism due to the detection of a stimulus.
Every living thing exhibits irritability. Even plants! Just because they are sessile (immobile and fixed to one place) and most of their responses don't usually occur within a time frame that is enjoyable or realistic to observe on your own doesn't mean that they don't happen.
For example, how do plants know which way to grow? They have to respond to the force of gravity, the direction of the sunlight, the location of water and touch in their growth patterns. These mechanisms of growth by which plants adapt to environmental changes are known as tropisms. Tropisms can either be negative or positive; negative tropism is growth away from a stimulus while positive tropism is growth towards a stimulus.
Tropisms are the result of differential growth, which is when cells on one area or organ of a plant grow at a faster rate than another area. This usually happens through asymmetrical elongation of cells on either side of the responding organ. Because plant cells are inseparable due to their cell walls, they cannot move relative to one another. Therefore, the side that grows faster due to faster elongating cells forces the organ to bend towards the slower growing side. This differential growth happens due to auxins (which CSEC doen't require to understand in detail). Auxins are plant growth hormones that are distributed asymmetrically to cause some parts to grow faster than others, resulting in the organ growing in a certain direction. In the shoot, auxins cause faster growth, while in the roots, they inhibit growth.
There are several types of tropism:
Phototropism
This is the growth of the plant in response to light. This is a form of positive tropism, since plants will grow towards light. Auxins will move away from the light- so the part facing away from the light will grow faster. This results in the plant growing towards the light.
Thigmotropism
Thigmotropism is the growth of a plant in response to touch or contact with an object. This pattern of growth is demonstrated by climbing vines, which have structures known as tendrils. When the tendril makes contact with an object, sensory epidermal cells on the surface of the tendril are stimulated. The auxins will accumulate on the side of the tendril not in contact with the object, which causes tendril to coil around the object. The twining of the tendril secures the plant (which usually has a herbaceous/non-woody stem) to the object to gain support.
Geotropism
Geotropism, or gravitropism is the directional growth of a plant based on the force of gravity. This is obviously very important to the orientation of the plant, since it is geotropism that causes the shoot to grow downwards against gravity (negative geotropism) and the roots to grow downwards with gravity (positive geotropism).
How do auxins play into this? Well, if you were to take a plant and lay it on its side:
Through this mechanism, the roots still grow downwards and the stem re-orients itself upwards through its growth.
Apart from auxins, amyloplasts (also known as statoliths) are specialized plastids that contain starch granules and settle downward in response to gravity. Amyloplasts are found in shoots and in specialized cells of the root cap. When a plant is tilted, the statoliths drop to the new bottom cell wall. A few hours later, the shoot or root will show growth in the new vertical direction.
Hydrotropism
Hydrotropism is the growth of a plant in the direction of water. Plants will exhibit either positive hydrotropism to find water in protection against droughts, or negative hydrotropism to avoid areas over-saturated with water. The cells on the side of the root closest to the water source experience slower growth than those on the opposite side.
(If you want to read a more advanced explanation of tropisms, here is a magazine article and a Thoughtco article)
Many plants also exhibit chemotropism (growth in response to chemicals) and thermotropism (growth in response to temperature).
Even invertebrates display irritability, as they will move in order to find more ideal environmental conditions.
For example, if an invertebrate, like a woodlice, is placed in a choice chamber with one side moist and the other side dry, or one side exposed to light and the other side in shade:
As you can see in the above video, the woodlice will choose to go to a more moist and dark area, since this will keep them from drying out (dessication).
As we have already discussed, a stimulus invokes a response in living organisms. The stimulus is detected by a receptor and the response is carried out by an effector.
So, anything that detects stimuli (changes in the environment) is a receptor. Your sense organs are all receptors that detect specific changes.
Plants also have receptors, such as the aforementioned amyloplasts (sensitive to gravity), and the meristematic regions on the tendrils of thigmotropic plants (sensitive to touch).
Any organ that executes a response to a stimulus detected by a receptor is an effector. Your muscles and glands are effectors. You will pull back from touching something hot because your skin detects heat and triggers a reaction for your muscles to contract and pull your arm back. Plants also have effectors; their leaves, petioles and roots.
Why is the response to stimuli important?
This response to stimuli is necessary to an organism's survival because it allows it to stay in balance. By detecting changes in their internal and external environments and responding accordingly, organisms can adjust themselves to ensure their survival. Responding to internal changes is how living things maintain homeostasis, a state of steady internal, physical, and chemical conditions. It also allows organisms to adapt to their external environment and find conditions necessary to maintain life, such as how the hydrotropism of plants allows them to grow in the direction of water.
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