THE EARTH in its great bounty gives life to 500,000 different species of plants. Humankind in its wisdom has chosen to make 95 percent of its diet out of only 30 of these and millions of persons have starved to death as a result.

Researchers now tell us that there is no excuse for such myopia.We are on the verge, they say, of expanding the farm to include a dozen odd-sounding food plants as good or better for us as anything we now have: the winged bean, the buffalo gourd, the amaranth, the tamarugo.

Similarly, we have depended on the fossils of long-dead plants for our petroleumn; we have slaughtered a great sperm whale every 39 minutes to lubricate our heavy industry; we have denuded much of the earth in cutting down trees for firewood that take 20 years to grow again.

There is no excuse for these offenses either, we are told, now that we have rediscovered the leucaena tree that grows 20 feet high in two years, the jojoba desert bush that makes a perfect lubricating oil, the guayule natural rubber shrub and the scrawny euphorbia Lathyris weed that oozes what will pass for petroleum. We can grow a new future, the researchers promise, if our agricultural institutions have not plowed themselves into ruts too deep for change.

That may be a sizeable "if." "Anything that represents initiative in this area has to fight really hard because it's so damn difficult to kill some of the established things," said Rep. George E. Brown Jr. (D-Calif.), dogged sponsor of recurrent legislation to boost the funding for jojoba and guayule research. Scientists complain that they have vastly inadequate support in their hunt for the miracles they know are waiting.

The Green Revolution that exploded rice and other grain productivity by genetic tinkering had nothing to do with new plants but instead increased the world's reliance on the old familiar favorites. Fully three-fourths of all human food energy and protein comes from eight grains: wheat, rice, corn or maize, barley, oats, sorghum, millet and rye, according to Noel D. Vietmeyer, the acknowledged expert in the new crops field. Three of those grains - wheat, rice and corn - now make up three-fourths of all the cereals eaten worldwide.

"We're not trying to counter the Green Revolution but just to enter a kind of second phase of expanding this dangerously small larder," Vietmeyer said in his plant-filled office at the National Academy of Sciences.

Vietmeyer, a chemist, and two assistants are the office of the NAS Advisory Commission on Technology Innovation's special panel that has searched out and begun to publicize 36 little-known plants they say have great potential for the future.

Their budget this year is $278,000, with no increase in sight, although AID and the president's Office on Science and Technology fund specific projects. The Agriculture Department was authorized to spend $25 million this year on new crop research, particularly jojoba and guayule, but didn't do it, Brown said. Overall agricultural research, which concentrates only on improving familiar crops, is budgeted at around $700 million and going down. But the study of the world's mystery vegetation has just barely begun.

"Well, we rely on hunches, hearsay, folklore or just random picking on interesting plant families" in discovering useful species, said John Rothfus, research leader of the Agriculture Department's Horticulture and Special Crops Laboratory in Peoria, Ill. Vietmeyer scans old research files and polls botanists and foresters as well, and has picked 400 interesting suggestions to include in two forthcoming books.

"Some of these plants could have careers as meteoric as the soybean," Vietmeyer said. Kudzu and Hyacinth

NEW PLANTS face extensive testing from scientists burned by the rampages of kudzu vines and water hyacinths. The leafy kudzu, brought from Japan for erosion control, is gradually marching Sherman-like across the South, swallowing whole buildings, toppling telephone poles and smothering trees. The water hyacinth, carried worldwide for the sake of its lovely flowers, grows at such a fantastic rate that 10 plants can smother an acre of water under 600,000 plants in less than eight months. Where it goes, fish die, mosquitoes flourish and shipping halts. It has never been eradicated anywhere.

"In order for anybody to want to plant a new crop it's going to have to make them money, to do as well or better than crops they have now," Rothfus said.

Jojoba oil was sent as free samples from Vietmeyer's office to 100 companies that use fine lubricating oil, and their enthusiastic response led to a growing farmer interest. So hot is the item now, selling at $8 a pound, that California Gov. Jerry Brown was keynote speaker at the third annual Jojoba Conference in Riverside, Calif., last week.

Although agribusiness has millions of dollars wedded to the traditional crops and their marketing patterns, many of the new crops not only can but must grow in desert or jungle areas that are now unusable. Further, they could be grown by underdeveloped nations rich only in that kind of land.

Israel has invested $10 million in jojoba plantations on the Negev Desert; the buffalo gourd is being grown in Mexican wastelands and the sands of Lebanon, producing protein-rich seeds, fountains of polyunsaturated oil and mammoth roots that are 50 percent starch. The winged bean, with edible pods, protein-rich beans, excellent oil, starchy potato-like roots and foliage that appeals to cattle, grows only in wet, hot climates.

"The field is wide open," said Rothfus. "There are so many plants, and then, when you think of what can be done to improve them genetically, well, it just boggles the mind."