In many other cases has the light to pass plastids with all their different pigments. This is, for example, true for Volvox species, where one colony consists of a large number of single cells able to move in a co-ordinated fashion. The chloroplasts of Volvox cells are characterized by a specific, bowl-like shape and a posterior position within the cell. As a consequence is light that enters the cell via the posterior pole always filtered. The direction of the light can thus be determined by each cell.
Energy is required that can be supplied by photosynthesis.
The clearing unit between incoming signal and energy supply that could also be called processor or effector is still a black box to us. It was indeed possible to simulate a course of movements largely identical with the phototactic reaction of Euglena .