Two very good books have been published recently. One is about miniature orchids, the other a paperback edition of a book that explains the why and how of plant life and makes other books more useful and helpful.

"Miniature Orchids," by Rebecca Tyson Northern, (Van Nostrand Reinhold, 189 pp., well illustrated, $26.95).

Northen has been growing orchids for 37 years. With her husband, she has traveled extensively in Central and South America studying and collecting orchids in the wild. She has lectured at several world orchid conferences and has written several books about them. In 1979, Northen was awarded the Gold Medal of the American Orchid Society for her contributions to the horticultural literature.

"Miniatures are especially advantageous where space is limited," she says. "They contribute spice and variety to any collection and are no more difficult to grow than are ordinary orchids.

"In fact, there is no more mystery about growing orchids than there is any other group of plants, cacti, succulents and ferns, for example. Anyone who wants to try orchids can just as well start with miniatures as with larger plants.

"It has long been proven that orchids can grown on windowsills and under lights, as well as in greenhouses.

"Some reward the grower with jewel-like flowers as translucent as glass, others with blossoms like brightly colored carved wax; some with sprays of many small blossoms, others with flowers as large as the plants themselves.

"For those who have not grown orchids before, this book describes their basic needs and ways of caring for them and discusses common problems and their solutions.

"From among the hundreds of miniatures described in the book, many of great charm are available from commercial growers. Some are rare and one may have to search for them; others are not yet available in this country.

"Small orchids far outnumber the larger kinds, and are just as beautiful as their more conspicious relative. They are not always easy to find in the wilds, particularly when not in flower.

"The astonishing shapes of some are to be marveled at -- flowers with tails five times as long as their other parts, or with wild fringes, whiskers or knobs.

"And the colors! The orchid world is known for a fantastic array of colors, and the miniatures have every possible range and combination.

"Many are fragrant; some strongly so; others delicately or deliciously; and a few unpleasantly. They give forth their perfume at various times of the day or evening according to their habits developed through evolution and coordinated with those of their pollinators. Sometimes the fragrance is emitted steadily, sometimes in intermittent bursts, perhaps more likely to provoke the interest of passing insects.

"Typical forests, whether of low, warm or of high, cool elevations, abound in many kinds of epiphytic plants. Orchids share the trunks and branches of trees with mosses and lichens, ferns, aroids, gesneriads, philodendrons, peperomias, anthuriums and bromeliads.

"Sadly, tropical forests are being devasted in the name of expanding civilization, some through utter vandalism. In some countries there is little virgin forest left. Conservationists and government are working on the tremendous problem of how to save the forests."

"The Why and How of Better Gardening," by Laurence Manning (Van Nostrand Reinhold, 239 pp, illustrated, $4.95 paperback).

"In no sense is this a textbook on botany," says the author. "You can skim through it in about four hours reading time, although it covers much the same ground as a course at college. There is an enormous difference in treatment of the same basic facts. Merely to show in the simplest possible way what those facts are, to explain what sort of creature a plant really is, these are the aims of this book. It is written solely for gardeners. If it helps them understand how to garden more intelligently, it has served its purposes."

Sixteen sciences are involved in the study of plants, Manning says: systematic botany, embryology, morphology, plant physiology, ecology, geology, meterology, soil chemistry, pedology, soil biology, agronomy, plant pathology, plant genetics, agriculture, horticulture and forestry.

It is the purpose of this book to synthesize into one orderly story all the significant facts in these 16 sciences, says Manning.

In general, a plant is not like an animal, a single organism for all practical purposes. A plant is three organisms -- a trinity of root, leaf and stem. The root mines, the leaf manufactures and the stem transports. Unlike animals, plants have no definite term of life. There is no reason why trees should not go on living forever, except for the accident of storm or drought. Many trees are known to be more than a thousand years old.

A piece of flesh torn from any warm-blooded animal becomes a dead thing, while a similar piece of vegetation can grow into the whole plant structure from which it was taken.