The New Student's Reference Work/Phototaxis
Phototaxis (fō-tō-tăks′ĭs) (in plants), the sensitiveness of an organism, free to move about, to unequal illumination (see Irritability), to which it responds by taking up a definite attitude with respect to the direction from which the brighter light acts. Only water-plants are free to respond. Diatoms, desmids, some filamentous algæ and zoöspores of algas and fungi show these reactions. Algal zoöspores swim toward light, and fungous zoöspores swim away from it.