The movement of Euglena towards the source of light is a?

Step 1: Analyze the question The question asks what type of response is shown when Euglena, a single-celled organism, moves towards light.

Step 2: Explain the concept Plants and simple organisms respond to stimuli in different ways:

• Tropism (tropic movement): A directional growth response, usually in plants and fixed in place. Examples: a shoot growing towards light (phototropism), or roots growing downwards (geotropism). The organism bends or grows but does not move its whole body from place to place.
• Taxis (tactic movement): A directional movement of the whole organism from one place to another, towards or away from a stimulus. It is common in free-swimming unicellular organisms. Examples: Euglena, Chlamydomonas, and sperm cells moving towards chemicals.
• Nastic movement: A response of a plant part to a stimulus where the direction does not depend on the direction of the stimulus. Example: the folding of Mimosa pudica leaves when touched.
• Kinetic movement (kinesis): A non-directional change in the rate of movement or turning in response to a stimulus.

Step 3: Apply to Euglena Euglena is a free-swimming organism that uses its flagellum to move. It has an eyespot (stigma) that detects light, and it swims towards the light source so its chloroplasts can photosynthesize. This is:

• directional (towards the stimulus),
• a movement of the whole organism from place to place.

This is phototaxis, specifically positive phototaxis, which is a type of tactic movement.

Step 4: Answer The correct option is B. tactic movement.

Tip: Remember the difference this way:

• Tropism = Tree/Turn (growth): fixed organisms bend or grow.
• Taxis = Travel: the whole organism travels.
• Nastic = Not directional.

If the question says a free-moving, single-celled organism moves towards or away from a stimulus, think taxis.