Plants don't always seem particularly charismatic, but hidden from us in their slow-motion and chemical activities are incredible mechanisms that sense and respond to the world around them. Plants move in response to light, bending and stretching to get maximum sunlight. This phototropism extends all the way down to much smaller photosynthetic organisms. Like other bacteria, many species of photosynthetic cyanobacteria can swim and swarm to move towards areas where more food--sunlight--is available. Shading some regions will quickly clear them of bacteria, reproducing which areas were bright and which were dark, creating a living photographic image.
This photographic motion can be demonstrated at incredibly high resolution in a petri dish, "developing" a negative of a photograph of the Freiburg Cathedral (above) or people's faces (left). Plants and cyanobacteria both move in response to light, but what about the chloroplasts--the descendants of free-living photosynthetic bacteria that power plant cells? These organelles have their own small genome and their own living response to light conditions. Usually the chloroplasts are spread out throughout the plant cell, distributing the light-absorbing power across the cell:
But in response to very bright light, the chloroplasts will rearrange themselves, pulling away from the illuminated cell wall to protect themselves from photodamage. In low light, the chloroplasts push up against the cell surface, arranging themselves perpendicular to the light to absorb the maximum amount of energy. These adaptations influence the ability of the plant to optimize its light absorption, and have a very dramatic effect on the cell that can be visible to the naked eye. Shading some regions from a bright white light will cause the cells to darken as the chloroplasts move to the surface, while cells exposed to the light will appear whiter. This is the opposite effect as seen with the cyanobacterial "photography," here a "positive" image is used instead of a photographic negative to create an image.
The chloroplast motion can create a range of green depending on the light intensity, allowing for temporary, high-resolution images to appear on the still-living leaf after less than an hour. Roger Hangarter, a researcher at Indiana University who studies the genetics of plant tropism and light-induced chloroplast movement, uses this technique to create amazing images on leaves, like this image of Norman E. Good, a photosynthesis researcher. His website, Plants in Motion, shows time-lapse videos of cool plant behaviors and includes instructions on how to make images on leaves yourself.
Living photography visually demonstrates incredible physiological processes happening inside the cell, and has been important to photosynthesis research since the mid 1800s (PDF). Early research on the production of starch in plants used iodine stains to visualize where starch was being produced in the leaf. Iodine turns starch dark purple (you can try this at home with cornstarch and iodine from a first aid kit), and parts of the leaf that were kept in the dark would be white while parts that were illuminated would be stained, showing that light was required for starch formation. This technique can create long-lasting and highly detailed images on leaves.
Living photography like this brings together art and science, turning leaves into art, plant scientists into visual artists, and artists who use these techniques into photosynthesis researchers. Artists Heather Ackroyd and Dan Harvey approach living photography in a slightly different way, creating beautiful and temporary canvases from patches of grass. To create these images, the grass is kept in a dark room, and a negative is overlaid on the grass. When the grass is illuminated, the regions shaded under the negative will yellow, while the patches that get the most light will stay bright green.
Synthetic biologists use genetic engineering to make a very different kind of living photography. The 2004 UT Austin iGEM team moved light-sensitive genes from a photosynthetic organism and connected them with E. coli genes that create pigments. These engineered bacteria form light responsive "pixels", and a petri dish coated with the cells is 100 megapixel per square inch film that turns black depending on where light is shined on the dish. Whether the pixels are engineered E. coli, cyanobacteria, chloroplasts, starch granules, or blades of grass, living photography illuminates what's happening inside living cells.