the movement of cells (zoospores, spermatozoids, leukocytes) and freely moving plants and protozoans in response to chemical stimuli. Positive chemotaxis is movement toward the source of the chemical stimulus (along the chemical concentration gradient in air or water); negative chemotaxis is movement away from the source. The phenomenon is known to exist in a number of microorganisms and invertebrates. The movement of insects in response to pheromones may also be regarded as chemotaxis.
The substances that induce chemotaxis vary from organism to organism. For example, cyclic adenosine monophosphate is an aggregating substance for soil myxomycetes of the genus Dictyostelium. Female sexual cells of the aquatic fungus Allomyces release the isoprenoid syrenin, a stimulus of chemotaxis of male sexual cells toward female cells. The mechanism by which a chemical signal is sensed (chemoreception) and the pathway to the corresponding physiological reaction, that is, oriented movement, are not thoroughly understood. Chemotaxis plays a part in the search for food, in fertilization in higher plants and animals, and in phagocytosis.