As the sun rises higher into the sky and our days get incrementally longer, I am thinking about plant sight. I'm not talking sight as you or I know it but rather their own unique brand of knowing where the light is and how to respond to it. Anyone that has ever grown plants will have undoubtedly recognized the way in which houseplants lean towards the nearest window or sunflowers track the sun's path through the sky each day with their blooms. Plants need the light and know how to respond to it but how do they do this without eyes, nerves, or a brain to process the world around them?
One of the first tantalizing pieces of evidence to this puzzle came from none other than Darwin himself. With the help of his son he carried out a series of experiments on seedlings using a candle lit room and rather ingenious methodology. They knew that seedlings naturally bent towards candle light so they were curious as to which part of the plant was responsible for this response. They cut off some of the seedling tips, covered the tips of some with light-proof caps, and covered others with transparent glass caps. There were also control seedlings as well as seedlings in which they only covered the stems, leaving the tips exposed. What they found was that only the seedlings with their tips cut off as well as those with light-proof caps didn't bend.
So, it appeared that the tip of the plant was where "sight" occurs, at least when plants are trying to figure out where the light source is emanating, however, this is not the full picture. Plants can also measure the length of day. Known as photoperiodism, many species of plants will regulate growth and flowering based on day length. Long-day plants will only flower when days are at their longest. The opposite is true for short-day plants. But the question remains, how do they know? Scientists quickly figured out that they could mess with this photoperiodism in the greenhouse by turning lights on in the middle of the night, a technique that is a boon to the horticulture industry.
Research into this revealed that different wavelengths of light have different effects. Blue or green light, for instance, does not do anything to upset a plants flowering schedule whereas red light does. Even stranger, the relative shade of the red light also has an effect. Shining a bright red light on a long-day plant in the middle of the night will cause it to flower while you can cancel this effect by shining dark red light right after. This may seem weird but it makes sense when you consider how these plants evolved.
It is not actually the length of day that plants measure, but rather the length of night. Shorter nights mean longer days, an excellent cue that the environment is favorable for flowering. By turning on lights in the middle of the night, you are effectively simulating short nights. In nature, plants receive bright red light when the sun is rising in the sky and dark red light as it sets. Bright red light activates chemical cues for flowering and dark red light turns them off. Only when the bright red signal is turned on longer than the dark red signal will the plants actually flower.
The chemical responsible for this "color vision" in plants is known as "phytochrome." Unlike Darwin's experiments, shining light on the tip of the plant has no effect on phytochrome. However, shining light on even a single leaf will elicit a response. Plants in which the leaves have been pruned will not react to red light at all. Though I can't speak for leafless plants like cacti, I am sure the concept remains the same, albeit more adapted to their lifestyle.
In total, roughly 11 photoreceptive compounds have been identified in plants. Though they do not perceive images as you and I do, their sense of "sight" is nonetheless quite sophisticated. Plants feed on light so it is no wonder that they have quite the chemical arsenal for responding to it.