BOTANY 101
Fetching Foliage The science behind autumnal beauty
by Daniel Murphy | illustrations by Lisa Spangler FOR A LEAF, BEING GREEN MEANS BEING AT WORK. Beginning in spring, when it first pushes out from a bud on a branch, a leaf is engaged in food production. The work is done within its cells where carbon dioxide from the air and water sent up from roots are turned into sugars in a sun-powered process called photosynthesis. For many of us, this was one of the first things we learned about plants in elementary school. Recall those illustrations of a plant in the sun, taking in carbon dioxide, giving off oxygen, and feeding itself in the process. Key players in photosynthesis are plant pigments called chlorophyll. They collect photons — also known as light energy — which are then used to power the production of food. Chlorophyll is unable to absorb green light, so the color green is reflected back, giving foliage its familiar color. Throughout the growing season, these pigments take a beating from the sun. Spent chlorophyll is regularly refreshed in order to keep photosynthesis going. At summer’s end, when temperatures cool and day lengths shorten, production of chlorophyll trails off. The lack of chlorophyll reveals a set of pigments that have been helping out with photosynthesis all along. Yet, being fewer in number, they have gone unseen. Some are carotenoids, pigments that give autumn leaves their orange and yellow colors. (If 8 | W I L DF LOW E R
you’re picking up on the similarity to “carrot,” that is no coincidence; beta carotene is a type of carotenoid, as well.) Shorter and cooler days signal additional changes in deciduous trees. Sugars are directed towards the roots, and the cells that make up sugar-transporting channels start to die. This process can trap sugars inside the leaf, which, in some species, synthesize into anthocyanins, pigments that are seen as shades of red and purple. The death of conducting cells where the leaf meets the branch eventually leads to leaf drop. During the life of a leaf, waste products stored within cells are made up of compounds called tannins — the same that exists in grape skins (and stems and seeds), giving certain wines a more dry, astringent character than others.