Figure 11.1. Bacterial flagella organized into a bundle during a run (left) and
unraveled during a tumble (right).
Another prominent environmental stimulus that can influence the
behavior of microorganisms, as well as plants and animals, is light.
The process of moving toward light is called phototaxis (in the case of
the entire organism moving toward, such as a swimming bacterium)
or phototropism (in the case of a bending or growing toward, such asa
plant near a window). In organisms that use light as an energy source
(photosynthesis), phototaxis and phototropism increase exposure. In
organisms that do not derive energy directly from light, phototropism
and phototaxis may enable movement toward a more open region in
order to disperse spores or seeds.
The fungus Phycomyces does not use photosynthesis but nonethe-
less has exquisite sensitivity to light, which it uses as a cue to direct
the growth of its fruiting bodies, called sporangiophores. Phycomyces
sporangiophores typically have stalks that are several centimeters
long. At the tip of each stalk is a structure called a sporangium—about
0.5 mm in diameter and containing around a hundred thousand