A Little Botany
In order to understand how plants respond to light, it's important to understand how they function. All plants have "vascular" systems--biological structures that carry nutrients, digest and then transport the results to their final destination where they will be used to form new plant tissue, flowers and fruit. The pathways of the vascular system run up and down the plant's branches, one (xylem) carrying water and nutrients like nitrogen, calcium and manganese up from the roots and another (phloem) carrying complete food. A third pathway (cambium) is where cells divide and tissue forms. These pathways may travel in rings, as in a tree, or in bundles, as in a stalk of celery. The intersection of these systems takes place at the limits of the system in the plant's leaves. As with any highway, however, no movement could occur without a form of energy to move the goods.
How Plants Use Light
In a process called photosynthesis, plants gather sunlight through tissues, most efficiently through their wide, thin leaf surfaces. Plants adapt to "face" the sunlight, actually turning to present more leaf space toward the light. Energy absorbed is used to split water molecules brought up from the roots and carbon dioxide that filters into the leaf through "stomates," passages similar to pores in skin. Many plants harvest carbon dioxide and sunlight at the same time, but plants in hot or dry climates--cacti, succulents and persistent weeds like crabgrass--have adapted so that they can open stomates to collect carbon dioxide at night and close them during the day, processing the previous night's carbon dioxide harvest. Some plants grow thick outer skins to protect the chloroplasts that lie between the top and inner levels of the plant's skin and look very different from those that grow in more moderate climates. Some adapt by altering pigmentation to reflect more light or change the arrangements of chloroplasts, the pigmented cells that absorb infrared and ultraviolet light.
Adaptation, Respiration and Transpiration
Since photosynthesis is essential to plant life, many behaviors and adaptations of plants are based on responses to light. Plants often self-adapt to non-native lighting and temperatures by changing coloring or even reproductive patterns--seeds, bulbs, tubers and rhizomes are all adaptations within one family--the angiosperms. Photosynthesis is efficient but it does produce two waste products. One, oxygen, is expelled through the stomates following formation of the carbohydrates. The second, water, is cast off as it travels through the vascular system, lubricating tissue and moving food around in the plant. The first, respiration, helps clean our atmosphere and provide oxygen for us to breathe. The second, transpiration, is visible as "dew" that drips from plants in the cool of the morning.