Botanical treasure hunters ask: Have you seen a ghost?

They may never find the ghost orchid: the botanists who have nearly finished collecting the seeds of all Britain's wild flowers, plants and trees now feel there are some they will not be able to obtain.

They may never find the ghost orchid: the botanists who have nearly finished collecting the seeds of all Britain's wild flowers, plants and trees now feel there are some they will not be able to obtain.

Seeds from more than 1,300 of the 1,442 species of flora regarded as properly native to Britain have now been stored at the new £80m Millennium Seed Bank of the Royal Botanic Gardens, Kew.

Scientists at the seed bank, which will open its doors next month at Kew's outstation at Wakehurst Place, West Sussex, aim to collect seeds from all Britain's flora, which would be a unique conservation achievement. But as they approach the end of the task, with about 120 species to go, they feel that some will always remain beyond them.

The project relies on a group of 200 enthusiastic botanists, professional and amateur, who have spent the past three years scouring the country for seeds, from mountain tops and moorlands to lowland ditches and streams.

Steve Alton, the project co-ordinator, said certain wild plants were simply too inaccessible, too rare or too infrequent in appearance. The ghost orchid was a typical example. This plant, which Richard Mabey in the encyclopaedic Flora Britannicacalls "the most mysterious and secretive of all our orchids, prone to sudden manifestations and disappearances" is extremely erratic in its appearance and may not be seen for 15 years at a time. The plant spends nearly all its time underground in the deepest shade of beech woods, relying on nutrients from rotting vegetation rather than photosynthesis, and is devoid of chlorophyll - the substance that makes plants green - and is without leaves.

In the rare years when it did send up a flower spike, Mr Alton said, the only way to find it was to go into the beech woods and shine a torch horizontally along the leaf mould on the ground. "If something pale sticks up it may be a ghost orchid."

Richard Mabey said "only a score or so have been seen in Britain since records began" and Mr Alton thinks the chances of one of his seed collectors finding one are not high.

Other plants also present obstacles to the seed collectors, especially ones from the Scottish mountains such as alpine blue sow-thistle (on inaccessible ledges) and alpine rock cress (too few plants to take seeds safely). Certain other seeds, such as acorns from English oak won't last in the dry cold conditions ideal for most.