Science: Autumn's Chemistry

From hedgerow, lawn and wooded hill

Departs the summer's chlorophyll;

The elms and hickories lose their green

And glow instead with carotene,

While sumacs, maples redden in

A burst of anthocyanin—

And TIME is moved to tell its clients

The reason why in terms of science.

When the deciduous (non-evergreen) leaf begins to die and disintegrate, the molecules of its unstable green pigment, chlorophyll, disappear. Its departure reveals the yellow carotene and xanthophyll which have been present but masked by the green all summer.

Red appears in maples, sumacs and some other plants when slowdown of the trees' physiological processes prevents carrying away of the sugars (made with the...