Prostrate cells of Micrasterias thomasiana Archer were irradiated from above with intensive blue light. Many of the cells reacted by rising to a profile position. During a period of 15 to 90 min the response is linearly dependent on the duration of irradiation, inferred from the number of rising cells. In a range from 10 to 30 μmol m(-2) s(-1) (equalling 2.7-8.0 W m(-2) at a wavelength of 450 nm) and for an irradiation time of 30 min, the rising reaction was linearly dependent on the quantum flux density. Choosing 30 min irradiation time and a quantum flux density of 30 μmol m(-2) s(-1), the reactive number of rising cells was employed in establishing an action spectrum. As a result of this, a flavin is postulated as the light-percepting pigment in the reaction, whereas chlorophylls do not appear to be involved. The rising reaction can be distinguished from other light-induced movements as a strong-light response, resembling in this respect the movement of chloroplasts within cells. The different sensitivity of individual cells and the importance of this strong-light response for the algal cells is discussed.

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