Unless we act quickly and decisively, one-third of the plant species on Earth could go extinct within 50 years, Peter Raven, director of the Missouri Botanical Garden, said yesterday at a conference of the International Botanical Congress in St. Louis, Mo. Among Raven’s recommendations is the creation of a new political entity, probably under the auspices of the U.N., to monitor and preserve plants around the world. At the same conference, Oregon State University professor Jane Lubchenco presented findings that indicated humans have changed nearly half the Earth’s land surface, with devastating effects on other species. Lubchenco also said there are 50 dead zones in coastal waters with little or no oxygen, where most species cannot survive.