BIOLOGY Volume 2 - A Guide to General Biology - 2004
16. PLANT COORDINATION AND REGULATION
16.1. Plant Movements
16.1.2. Taxes
Taxes (singular: taxis) are directional locomotory movements of an entire Organism, an individual Cell, or an organelle, triggered and guided by a specific external stimulus. Like Tropisms, they can be positive or negative; furthermore, they can be classified According to the Nature of the stimulus (Table 16.2). Note that such movements are characteristic of all groups of organisms except plants and Fungi.
Class="center">Table 16.2. Examples of taxes
Stimulus |
Type of taxis |
Examples |
Light |
Phototaxis |
Positive phototaxis: Euglena (a unicellular alga) swims towards light, METABOLISM/14.html">Chloroplasts move towards light, fruit flies fly towards light Negative phototaxis: earthworms, blowfly larvae, woodlice, and cockroaches avoid light |
Chemotaxis |
Positive chemotaxis: spermatozoids of liverworts, mosses, and ferns swim towards substances released by the egg cell; motile Bacteria move towards a food source Negative chemotaxis: mosquitoes avoid repellents |
|
Air (oxygen) |
Aerotaxis (a type of chemotaxis) |
Positive aerotaxis: motile aerobic bacteria move towards a source of oxygen |
Gravity |
Geotaxis (gravitaxis) |
Positive geotaxis: planulae (larvae of certain Cnidarians) settle on the sea floor Negative geotaxis: ephyra larvae of certain cnidarians swim upward toward the sea surface |
Magnetic field |
Magnetotaxis |
Certain motile bacteria |
Current |
Rheotaxis |
Positive rheotaxis: planarians crawl against the current; moths fly against the wind |
16.3. Euglena and Chlamydomonas are unicellular Algae that swim using flagella. They exhibit positive phototaxis, i.e., a tendency to move towards light. This is beneficial for both Euglena and Chlamydomonas, as both algae rely on Photosynthesis for survival. Consider how you could design an experiment to determine the light intensity preferred by Euglena or Chlamydomonas.
16.4. Fig. 16.1 shows the distribution of motile bacteria 10 minutes after being placed in a drop of Water under a coverslip, alongside a fragment of a filamentous alga, such as Spirogyra.
a) Propose a hypothesis to explain this distribution of bacteria.
b) How could you test your hypothesis?
Fig. 16.1. Distribution of motile bacteria on a Microscope slide.
Last update: 06/08/2026
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