Autumn's changes in leaves and grasses, says Dick Hildreth of Salt Lake's Red Butte Garden, "are a combination of weather, physiological and chemical processes in the plant and some anatomical pro-cess-es too."

The changes result from a progression shorter days and cooler nights, Hildreth says. As a result of these, abscisic acid forms, damaging cells at the bases of the leaves. Photosynthesis halts - foods cannot pass from the leaves into the stems. Chlorophyll, the green we see, is destroyed. Colors already in the leaves emerge while additional pigments are sometimes created."And all of this is, of course, influenced by what has happened to the tree, in some cases over the preceding seasons," he says: the depth of the previous winter's snowpack; the amount of summer precipitation; and, finally, impact of autumn rains.

"The ideal situation," Hildreth says, "would be bright, clear, somewhat warm days followed by cool nights but not freezing (hard freezes will destroy all color), adequate moisture and then a prolonged process of Indian summer."