Taxis, Tropism, and Kinesis.

In biology, all of the above are responses to stimuli. We hear these words bandied about every now and again, and it can be rather confusing when they are. What are the essential differences?

1. Kinesis refers to an organism's non-directional response to a stimulus, such as light-intensity or humidity. This is subdivided into two moieties: Orthokinesis, or the degree of movement based on the intensity of the stimulus, and Klinokinesis; the rate at which at an organism turns in response to a stimulus.

To help grok Orthokinesis, I like the word 'frenetic' for the associations it conjures. Frenetic is 'fast and energetic in a rather wild or uncontrolled way' (ODE). Think of the evening before a major writing assignment is due. As the hours pass, you feel more freneticism, and begin pacing around the house. Add some coffee, and you pace more so. This movement is 'kinetic' in that while it is a response to a stimulus (here, the stress of deadlines), it does not function to address that stimulus.

I admit I don't really have a metaphor to help with Klinokinesis. You probably get it though.

2. Taxis differs from kinesis in that it is focused: the organism either moves away from a stimulus or towards it. Also unlike kinesis, which refers both to motility and turning, taxis only refers to the former.

3. Tropism is the turning of an organism in a specific direction in response to a stimulus. The classic example is of sunflower buds that orient themselves to face the sun (heliotropism). Another example of tropism is phototropism: movement in relation to light.

Kinesis receives the most attention as it includes both motility and orientation, while taxis involves solely motility and tropism solely concerns orientation.


What's the lesson from all this?

Well, when the 48th-billion-Marvel-superhero-flick gives you examples of telekinesis or pyrokinesis, you can tell your friends that the filmmaker is lying. No one would want pyrokinesis, since that would involve flames exploding in random direction with no oversight.

What about pyrotaxis? Would that be better, or would it just involve jumping into flames? Mind you, superheroes do that anyway...