The continents, according to two scientists at the U.S. Geological Survey, are growing by a total of more than eight-tenths of a cubic mile every year.

Most of this growth in volume comes from parts of the sea floor that are added to the edges of continents as the great drifting land masses -- each a vast plate of rock jostling over the Earth's surface -- rub against or collide with the ocean floor plates.

The material that continents acquire from the sea floor consists of lava from undersea volcanos, minerals deposited by marine organisms and additional sediment that has eroded from the continents and washed into the sea.

The geologists calculate that the mass added to the continents annually is about 1.9 cubic miles but that at the same time a little over one cubic mile of continental mass is washed into the sea. The difference is a net growth of 0.84 cubic miles.

The findings are published in Science by David G. Howell and Richard W. Murray of the Geological Survey's Menlo Park, Calif., office. Murray is also on the faculty of the University of California at Berkeley. Greenhouse Effect: Good News for Plants

In the cloud of worry about Earth's developing "greenhouse effect," scientists at an Agricultural Research Service laboratory in Phoenix have found a silver lining.

The greenhouse effect is a result of increasing concentrations of carbon dioxide in the atmosphere, which allows sunlight to reach the Earth but retards the resulting heat from radiating back into space.

The amount of carbon dioxide in the air is expected to double slowly over the next century, causing a general climatic warming.

But because plants need carbon dioxide, using it with water to carry out photosynthesis, many species are expected to grow more luxuriantly as concentrations of the gas increase. Botanists are concerned, though, that an increased water supply might be necessary to support the increased growth.

Experiments in a glass greenhouse with an artificially doubled carbon dioxide level in the air, however, show that at least one plant, cotton, produces much higher yields without requiring increased water. According to Bruce A. Kimball at the Arid Zone Crop Production Research lab, a branch of the Agricultural Research Service, other plants may have the same ability.

It may be warmer in the decades to come, but it could also be greener. Immunizing Elms: Another Step Forward

Another step toward creating an elm tree that is immune to Dutch elm disease has been achieved by researchers at an Agriculture Department lab in Delaware, Ohio.

The researchers have succeeded in coaxing whole American elm trees to grow from individual leaf cells. Although the trees grown from cells are still vulnerable to the disease, it will now be possible to fuse such cells with similar cells from elm varieties that are immune to the disease.

Cell fusion combines the genes of both varieties. Research on other plants has shown that as the fused cell develops, it discards genes randomly, leaving new combinations of genes.

The Ohio researchers plan to make many cell fusions, stimulate them to develop into trees and look for individuals that combine the highly prized shape of the American elm with the disease resistance of some other elms. Design Challenge: Monument in Space

An Eiffel Tower in space -- or something like it -- is the goal of a French organization sponsoring a contest to design a structure that could be launched into orbit and be visible to the naked eye as a symbol of "universal communication, the great dream of our times."

The sponsor is Paris' Eiffel Tower Company, which says it is seeking a design for the space age that is "as audacious and imaginative as that of Gustav Eiffel for the age of steam." Eiffel also was the engineer of the Statue of Liberty.

According to the contest rules, the satellite must have no military or commercial purpose. Peaceful or scientific uses are permitted.

Judging, in October, will be by a panel of scientists, artists and space engineers. First prize is $20,000. There is a junior category for entrants under 15 years of age.

Apply to La Tour Eiffel de l'Espace, Champ de Mars, 75007 Paris, France.