<P> The eyespot apparatus (or stigma) is a photoreceptive organelle found in the flagellate or (motile) cells of green algae and other unicellular photosynthetic organisms such as euglenids . It allows the cells to sense light direction and intensity and respond to it by swimming either towards the light (positive phototaxis) or away from the light (negative phototaxis). A related response ("photoshock" or photophobic response) occurs when cells are briefly exposed to high light intensity, causing the cell to stop, briefly swim backwards, then change swimming direction . Eyespot - mediated light perception helps the cells in finding an environment with optimal light conditions for photosynthesis . Eyespots are the simplest and most common "eyes" found in nature, composed of photoreceptors and areas of bright orange - red pigment granules . Signals relayed from the eyespot photoreceptors result in alteration of the beating pattern of the flagella, generating a phototactic response . </P> <P> Under the light microscope, eyespots appear as dark, orange - reddish spots or stigmata . They get their color from carotenoid pigments contained in bodies called pigment granules . The photoreceptors are found in the plasma membrane overlaying the pigmented bodies . </P>

What is the function of eye spot in euglena