Saturday Science: Earth: A Biography: A Sidebar: Autumn Color

Those of us who live in the north temperate zone are treated every fall to autumn color, when the leaves on the trees turn from green to red, orange, yellow, and everything in between. This is because the chlorophyll in the leaves, which gives them their green color, breaks down, exposing other pigments that have been hiding under the surface.

Chlorophyll is one of a class of photoshymthetic pigments that first made their appearance roughly three billion years ago. There are others -- look at any number of decorative plants and shrubs, which show red, yellow, orange and various combinations of color. And they all enable photosynthesis, which not only allows plants (and some bacteria) to make their own food, but produces as a byproduct the oxygen in the air we breathe.

No one is quite sure how some bacteria -- known officially as "cyanobacteria" -- first made use of photosynthetic pigments, but they did. One of the immediate results was oceans full of rust, which sank to the bottom and created what are nown as "banded iron formations" -- the source of mose of our iron ore. Once that was accomplaished, the oxygen that these bacteria were producing escaped into the atmosphere -- which at that point was composed of nitrogen, carbon dioxide, and water vapoor - and created the ozone layer, which proved to be of great significance: the ozone layer filters out most of the harmful radition from the sun and enabled life to move from the oceans to the land.

Scientists believe that land plants are descended from green algae, mostly because they contain the same kind of chlorophyll. It actually makes a lot of sense -- who has not seen a clump of algae on a shore, periodically left high and dry as the tide goes out. And the first land plants were mosses and selaginellas, which require a moist environment.

But back to fall color: What we're seeing after the chlorophyll breaks down is the other pigments, which will last until the leaves fall. There's even an order in which this all happens, at least as far as the trees themselves are concenred: here in Chicago, the firt to turn are the locusts and ashes, followed by maples and basswoods. Mixed in there are birches, ginkos, and the various others that get planted in the city. Oaks are notable for having prsisent leaves -- they turn color and die, but they don't fall. That's probably why squirrels pefer them for nest-building: the leaves hold to the twigs so you bedroom isn't constantly falling apart.

So that's all I have to say for now about fall color and how it fits in which the scheme of things.