To the left: Cross section through a cell with photoreceptors that are localized close to the surface (pink dots) whose density is a measure for the relative strength of absorption. To the middle: resulting absorption gradient (light > dark). To the right: orientation of the chloroplasts within the gradient. From top to bottom: Shading principle (Equisetum spore), collecting lens principle (Hormidium), principle of the "dead space" (Vaucheria), principle of dichroitic orientation (Mougeotia; W. HAUPT, 1977)