Timothy walker is drawn to the budding artists of biology

'The verisimilitude of Vicky Cox's White Onion and Skin makes you reach for the knife and the chopping board'

a new flowering - 1,000 years of botanical art

Ashmolean Museum, Oxford, to September 11

In a garden, science and art meet on even terms; in botanical art, they lock horns. The outcome of the contest can be either a clear victory for one side or other or a satisfying draw.

This conflict between art and science is very clear to see at the exhibition of 1,000 years of botanical art at the Ashmolean Museum. At its core is the remarkable Shirley Sherwood Collection, amplified and augmented by works from Oxford University's equally rich holdings, particularly those from the department of plant sciences. The combination of these treasures is a display that provides the perfect introduction to the history of botanical art. It could not be bettered by any other university, not least because some of the pictures are accompanied by the original specimens from the Herbarium.

The subjects of the works on show are frequently beautiful living objects that require neither improvement nor embellishment. Yet some artists see the need to clean up the appearance of the plants, thus rendering them sterile. This is not an accusation that can be levelled at Patricia Arroxellas Villela's exuberantly scruffy Strelitzia enfurta . This remarkable picture captures both the botanical detail of the plant and its character, but therein lies the problem: is this a piece of botanical illustration or is it botanical art? Are the two mutually incompatible? Not all artists are able, or attempt, to combine both disciplines. The work of Ferdinand and Franz Bauer stands out in this respect, but it is a joy to see that Jenny Philips's Corymbia ficifolia perpetuates the tradition of combining accuracy with artistry.

The challenge to the artist is to capture the primary essence of the subject, but the subjects are living so they have a soul; conveying this in a two-dimensional picture is very difficult. For example, when depicting fruit or vegetables the image should make your mouth water. Sheila Siegerman's exquisite Litchi chinensis succeeds where most portraits of bananas in the exhibition fail. Similarly, the verisimilitude of Vicky Cox's White Onion and Skin makes you reach for the knife and chopping board.

The function of flowers is to facilitate sexual reproduction. The attraction of pollinators exploits both the visual and olfactory senses, with taste and touch playing minor roles. Conveying these aspects in a piece of two-dimensional art is another problem. A taxon that has taken sexual reproduction beyond the acceptable limits of deceit and trickery yet lends itself perfectly to botanical artistry is the Orchidaceae. The unnatural perfection of orchid flowers enables many artists to capture their dull beauty, yet James Bateman's Stanhopea tigrina also oozes scent.

At a time when photography is undergoing a revolution and when digital imaging enables many people to create perfect, blemish-free pictures, why do we need botanical artists? Some undergraduates, and some professors who should know better, criticise biology practicals where they are required to draw plants and animals. "Why can we not take a photograph with a digital camera and label that?" they ask.

The reason is simple: until you have drawn a specimen, you have not looked at it properly. Until you have seen this exhibition, you have not looked at botanical art properly.

Timothy Walker is director of Oxford University's Botanic Garden and Harcourt Arboretum.