Few kids can resist collecting the kaleidoscopic leaves of autumn. Each fall, as the days grow shorter, decreasing daylight triggers the change from quiet greens of summer leaves to loud yellows, oranges and reds—a transformation that inspires little ones to gather nature's handiwork into bouquets for parents, grandparents and teachers.
Although not all leaves turn red, those that do contain three types of pigment: chlorophyll, carotenoids and anthocynins. During the growing season, leaf cells contain chlorophyll and carotenoids. Chlorophyll is the green chemical that helps leaves convert sunlight, water and carbon dioxide into sugar. (Sap carries the sugar to other parts of the tree to help it grow.) The green is so powerful it covers the yellow, orange and brown carotenoids, but most anthocynins (red pigment) are produced in autumn when the days are still fairly long and the leaf cells are chockfull of sugar.
When there's less sunlight, leaves use less and less chlorophyll, and the green chemical begins to break down. Suddenly—or so it seems—we can see the bright orange and yellow carotenoids and the red anthocynins.
Some years the leaves are more colorful than others. For truly spectacular displays, trees need lots of rain during the growing season, followed by an autumn with warm days and cool nights.
Separating the colours
Here's an experiment that separates the chlorophyll from the yellow/orange carotenoids in a green leaf. (One word of caution: isopropyl alcohol is poisonous, so don't let your child taste it or inhale the fumes.)
Put two tablespoons of green leaves torn into tiny pieces into a small baby food jar and pour 125 millilites rubbing alcohol over top. Using the back of a metal spoon, mash the leaves, then let the mixture sit for about 15 minutes. The alcohol will turn green. Meanwhile, cut a 3- by 10-centimetre strip from a paper coffee filter and wrap one end around a pencil; secure it with tape. Lay the pencil over the jar's rim so the filter paper just reaches the alcohol inside the jar.
The liquid will begin to move up the strip. Leave the end of the strip in the liquid for 30 minutes(or longer for more colour), then place the strip on a paper towel to dry. At the top of the strip just before the paper bends around the pencil, you'll see bands of green chlorophyll and the yellow/orange carotenoids.