Some of the finest gardens in the world are in Great Britain. There are many things that contribute to making them great, according to Graham Stuart Thomas, gardens consultant to the National Trust of Great Britain, including the state of upkeep, the care in planting, bird song, the blended colors and fragrance of plants, and an endless progression of mingled beauty.

Thomas is the author of a fine new book, "Great Gardens of Britain" (Mayflower Books, 292 pages, beautifully illustrated, $19.95).

The National Trust was founded in 1895. It was realized that the growth of population, the spread of industrialization and a lack of planning were rapidly spoiling much of the beauty of England.

To halt this uncontrolled destruction, to educate public opinion, and to give people access to the countryside, a body of responsible private citizens was set up to act as trustees for the nation in the acquisition and ownership of land and buildings worthy of permanent preservation.

For several years there was little progress. But its properties today include mountains and moorland, coastland and woods, commons and pastures, lakes, waterfalls, bridges and canals.

Its buildings include prehistoric and Roman antiquities, medieval chapels and castles, villages, cottages, mills, inns, barns and dovecotes.

It parks and gardens illustrate many different types and periods. Its country houses large and small, some of them houses in which famous people lived and worked, contain important collections of pictures, furniture, tapestry, books, sculpture, silver, china and musical instruments.

Before World War II, several great houses, surrounded by gardens of quality, came into the trust's care.

The war caused great havoc in gardens. After the war, owners of large gardens were faced with an impossible situation when, in perhaps their declining years, they were called upon to refurbish their gardens with probably only half the staff that they were able to afford in 1939.

It became obvious that many gardens would disappear, particularly those which, though of national importance, had not in their midst a house of like value which might be offered to the Trust.

The Trust evolved a plan for the acceptance of such works of art and botanical importance. A garden fund was established to take care of the maintenance of the gardens so accepted and to manage these gardens on behalf of the Trust.

In the early part of the book, Thomas traces the history of gardening to the present day, and includes all the important plant introductions which affected the gardens themselves. He discusses each of the gardens in detail, based on his own knowledge of them, providing a fresh insight into every aspect of gardening.

He gives his impression of each of the gardens, as well as those of other writers, with the donor's name, and date of acceptance.

"Today's gardening is an activity involving emotions and the exercising of choice," the author says, "but in early days, when many of the race went hungry, the idea of abundance was seen as part of paradise. For this reason the cultivation of fruits, herbs and vegetables was considered an approach to the ideal and there was no urge for artistry in the arrangement of plants in the gardens.

"But in countries of the highest civilization the beauty and form of plants gradually led to their artistic use; on the one hand was the area given to their cultivation for food, trade on study, and on the other the realms of artistry were measureless.

"Collections of plants form a reservoir from which we all benefit, so charitableness must come into our assessments of gardens; there is so much beauty in a plant that their possession can be everything to some poetic spirits, regardless of their placing."

"In our governing of gardens we admire all that nature gives us," he writes, "while at the same time holding her at bay. Is not this the essence of gardening? It is art allied to work. Directly we relax the picture fades, the lines become blurred and nature proceeds to 'surge softly back'. Man's noblest efforts of wood or stone or iron, if left untouched, will one day succumb to the forces of nature, forces which are gentle and soft in their remorseless creeping."

The book is illustrated with 150 black and white photographs, 50 monochrome drawings, 32 color photographs and 16 watercolors.