Until this month, the world’s premier collection of botanical art, consisting of some 200,000 drawings, watercolours, oil paintings and prints, had been kept filed away in drawers and boxes in the Library at Kew Gardens. A few pieces were exhibited from time to time in the Kew Gallery but never the rarer, more important ones. Now, with the completion of the Shirley Sherwood Gallery of Botanical Art visitors will be able to see what botanical art is all about.

The inaugural exhibition is a marriage of two collections: 68 items from Kew’s historic collection; and a further 116 from Shirley Sherwood’s personal treasure trove of some 700, mainly contemporary paintings. Sherwood started collecting only in 1990 when she saw, and fell in love with, a vibrant painting of a single purple orchid against a lush background of leaves by the British artist Pandora Sellars.

“People often say that botanical illustrations are all the same,” says Christopher Mills, the head of library and archives at Kew, “but the classic presentation of a single flowering stem, with roots attached, surrounded by pencil drawings and dissections of the various parts of the plant, emerged only after Linnaeus established that it was the sexual parts of the flower that would differentiate one plant from another.”

For the scientist this form of illustration still remains the most useful, and some regard the pencil drawing, with no colour infill, as the most accurate of all. But botanical art embraces a broader aesthetic and this show celebrates this richness, from the 15th-century engraving of a female mandrake to a contemporary interpretation of a giant bearded iris.

The exhibition moves through the five side rooms into the large main gallery, and the contemporary and historic paintings are linked mainly by their shared subject matter. Some are so lifelike – such as the spines on Christabel King’s cactus Cleistocactus fieldianus or Sally Keir’s pink rhododendron – that you feel you could stretch out and touch them. Others just take your breath away with their ability to capture the essence of the plant.

The Grand Masters of Kew’s collection includes works by Franz Bauer, whose eye for detail was so remarkable that, armed with just an elementary microscope, he drew a pollen grain in the 18th century that two centuries later was shown, by using an electron scanning microscope, to be absolutely accurate. Although Walter Hood Fitch may not be a household name, botanists have long appreciated his work, as he completed 10,000 drawings while working as the principal artist for Kew from 1837 to 1877.

Little is known about some of the works, such as the woolly thistle painted at nighttime (possibly by the German artist Johann Dietzsch), with an inky-black background offsetting the gossamer spider webs, moths and beetles resting on the leaves. The exhibition’s curator, Laura Giufridda, admits that one of her favourites – an early watercolour of cut tulips and double anemones – is by “artists unknown”.

Kew still commissions some 100 illustrations a year. Some of these will be new plants, others reinterpretations of existing specimens where scientists have discovered a new aspect of its structure that needs to be recorded visually, and a few will be records of plants that are endangered in the wild. Mills says there will always be a need for botanical illustrators. Many book publishers, he notes, who dropped illustrations from their natural history titles in the 1960s and 1970s in favour of photography, have returned to using them in their field guides.

The exhibition will leave you wanting to buy the book (Treasures of Botanical Art: Icons from the Shirley Sherwood and Kew Collections by Shirley Sherwood and Martyn Rix), to remind you of what you have seen and to help you find out a bit more about many of these elusive artists.

Shirley Sherwood Gallery of Botanical Art, Kew Gardens, Richmond (www.kew.org 020-8332 5655), until Oct 19, 2008

Editor's choice

• 70 facts you didn't know about Marvel
• The 50 Biggest Movies Of 2010
• The Bee Gees: 50 years on