Undated (AP) _ Today National Arbor Day is the last Friday in April, although many states plan observances to coincide with the planting season: South Carolina's Arbor Day is the first Friday in December, while Alaska deems the third Monday in May auspicious.

Exactly 100 years after Morton first exhorted his colleagues about planting more trees, John Rosenow - born, fittingly, in Elmwood, Neb. - was asked by then-Gov. James J. Exon to plan the state's Arbor Day centennial.

Rosenow, the new tourism director, was only 21 and fresh from the University of Nebraska when he accepted the task. He started the National Arbor Day Foundation with $10,000 and a staff of one - himself.

Today, almost 900,000 members make the foundation second only to the National Wildlife Federation as the nation's largest environmental organization. Members pay $10 to join, $15 annual dues thereafter, and receive, free of charge, 10 seedlings from 6 to 12 inches, in varieties suited to climate. More than 40 million trees have been given away since 1972.

Rosenow, now 40, the foundation's executive director since 1979, has spent half his life guiding it into an international environmental force. He has a theory on its success: ''The magic of planting a tree and caring for it is something each and every person can do to make a difference. It empowers people the same way a spark can light a bonfire.''

''Often that experience comes in childhood,'' he said, ''and it can touch off a tremendous, latent desire in people to make a positive contribution to the world they live in.''

When Christ walked the Earth, many forest-dwellers believed gods lived in trees. The Gauls worshiped oak. Sacred trees guarded entrances to Chinese villages. Egyptians left offerings of raisins to sycamores. American Indians believed each tree had its own spirit.

''When you think about tree rings, you have to have sense of reverence,'' said Rosenow. ''A 4,000-year-old sequoia may be one-third as old as civilization itself.''

Tree-huggers everywhere know the ditty by Joyce Kilmer:

''I think that I shall never see

A poem lovely as a tree ...

Poems are made by fools like me,

But only God can make a tree.''

Recite it on Arbor Day as you dig the hole. And then commit this, a poem entitled ''An Arbor Day Tree,'' author unknown, to memory:

''Dear Little tree that we plant today,

What will you be when we're old and gray?

The saving bank of the squirrel and mouse,

For the robin and wren an apartment house.

The dressing room of the butterfly's ball,

The locust's and katydid's concert hall.

The schoolboy's ladder in pleasant June,

The schoolgirl's tent in the July moon.

And my leaves shall whisper right merrily

A tale of children who planted me.''

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EDITOR'S NOTE - Tad Bartimus is the AP Mountain States Regional Reporter, based in Denver.