Magnolias are known to be among the most ancient of flowering plants. Their fossil remains have been found in rock formations of the Teriary Period dating back more than 100 million years. Throughout the intervening ages, during which other races of plants have evolved, magnolias have remained unchanged.

Magnolia flowers are of simple structure, with free carpels and numerous stamens, attached in spiral formation to a convex receptacle. Because there is usually no distinction between sepals and petals, these floral leaves are termed tepals and are arranged in whorls of three.

The open saucer-like blooms, this lack of differentation between sepals and petals and the spiral arrangement of their stamens represent the most primative form of flower structure. Apart from the tropical Magnolia coco there are no nectar glands, though the flowers of most magnolias are more or less fragrant, according to Neil G. Treseder, author of a fine new book, "Magnolias," published by Merrimack Book Service, 243 pages, well illustrated, $59.95.

Treseder represents the fourth generation of the well-known Cornish (Great Britain) nurserymen, established since 1820. During the 10 years he spent gathering information for the book, he studied magnolias in several Cornish gardens which contain many of the original introductions from such collections as Forrest and Wilson. He also visited magnolia collections in other parts of the British Isles, in America and in the Far East.

His book includes details of all Asian and American temperate zone species with the majority of their numerous hybrids and crosses recorded up to the date of going to press.

The many line drawings are by Marjorie Blamey and nine of the striking color plates are taken from her paintings.

When the magnolia flowers open, Treseder says, the reproductive organs become fully exposed and the stamens split open to shed their pollen, but by this time the stigmas have withered and become dry for they are usually only receptive while the flowers are closed. How then are magnolia blooms fertilized so that they set seed?

It has been found that they are largely pollinated by flower beetles which crawl between the overlapping tepals into the warm, sheltered environment of the flower chamber. It is probable that other insects also seek food and shelter there.

Beetles are known to have been plentiful in the Tertiary Period, millions of years before bees, wasps, moth and butterflies evolved, though they were preceded by dragonflies, mayflies, silverfish and springtails, Treseder says.

Dr. E. E. Leppik of the Plant Genetics and Germplasm Institute, U.S. Department of Agriculture, attributes the relatively slow evolutionary rate of magnolia flowers to the equally slow sensory development of their beetle pollinators.

Rhizobia are a type of nitrogen-fixing bacteria found in small knots on the roots of soybeans, alfalfa, clover and other members of the protein-rich legume family of plants. Scientists believe it may be possible to use them to boost crop yields on farms and in gardens to meet increasing world food demands.

The Department of Agriculture has established a special project at the Beltsville Agricultural Research Center, Beltsville, Md., to collect and study strains of rhizobia and distribute them to other laboratories where rhizobia investigations are under way.

Rhizobia have the natural ability to provide crops with nitrogen, says USDA microbiologist Deane F. Weber who will supervise the project.

"In their own way rhizobia fertilize certain crops," Weber says. "This happens without applications of commercial fertilizers, which require enormous amounts of natural gas to manufacture. Farmers simply coat seeds with rhizobia before planting to increase harvests."

Scientists in many laboratories now may be within reach of greatly increasing rhizobia's fertilizing abilities, Weber believes.

Since 1972 the cost of commercial fertilizers has skyrocketed. If prices remain high, as experts predict, commercial nitrogen fertilizers could soon be out of reach for farmers in lesser-developed countries.

To offset it, research is being increased to exploit natural systems of fertilizing crops. Rhizobia may provide crops with nearly twice the nitrogen as do bagged fertillizers.

The Beltsville project will be a dependable source of a wide variety of rhizobia strains which Wever hopes will keep research moving at near-optimum levels. Also, for areas of the world where farmers are not familiar with rhizobia the project will help spread the word on how to use current technology.

Rhizobia combine nitrogen atoms from air in the soil and hydrogen atoms from sugars in plant tissues to make ammonia, the basic ingredient of nitrogen fertilizer. This natural process is called biological nitrogen fixation. It often leaves fields more fertile after harvest than before the legume crop was planted.

When commercial fertilizer became cheap in the late 1950s research on rhizobia inoculation and the training of rhizobia technicians sharply declined, Weber says, and have never recovered.

Now when perhaps the need is greatest there is a troubling shortage of qualified scientists and technicians.

Countries where knowledge of rhizobia is in short supply can send technicians to Beltsville to learn how to isolate, purify and culture rhizobia suitable to particular growing conditions and how to make and apply rhizobia inoculum to seed.

In addition, Beltsville will provide guidance and advice to workers in lessdeveloped countries so that they can start their own rhizobia training programs.