Earth Day is April 22. Over the years, I have occasionally commemorated the event by quoting writers, philosophers and scientists who inspire love and respect for the planet. I do this to provoke thought and because words used wisely can change the world. This year I asked some friends for a few of their favorite quotations. Here are some of them:

Submitted by Roland Roth, retired University of Delaware ecologist-ornithologist:

"Earth provides enough to satisfy every man's need, but not every man's greed." -- Gandhi.

Submitted by Jim McCormac, naturalist, Ohio Division of Wildlife:

"Humanity must stop behaving like a Gengis Khan of the solar system and think of itself ... as nature's co-pilot." -- Edgar Morin, French sociologist.

Submitted by Todd Katzner, director of conservation and field research at the National Aviary:

"Growth for the sake of growth is the ideology of the cancer cell." -- Edward Abbey.

Submitted by Eriks Janelsins, director of Schrader Environmental Education Center, Wheeling:

"Live simply so that others may simply live." -- Ghandi.

"Look deeply into nature, and then you will understand everything better." -- Albert Einstein.

"Go outside!" -- Eriks Janelsins.

Submitted by E. D. Michael, retired professor, West Virginia University:

"Away from nature, Man's heart becomes hard." "Lack of respect for growing, living things leads to lack of respect for humans, as well." -- Lakota sayings.

Submitted by Penny Miller, director of Good Zoo, Wheeling:

"I think having land and not ruining it is the most beautiful art that anybody could ever want to own." -- Andy Warhol.

Submitted by Allison Wells, public affairs and marketing director, Natural Resources Council of Maine:

"The idea of wilderness needs no defenses, it only needs more defenders." -- Edward Abbey.

Submitted by Bill Thompson III, editor of BirdWatcher's Digest:

"After you have exhausted what there is in business, politics, conviviality, and so on, and have found that none of these finally satisfies, what remains? Nature remains." -- Walt Whitman.