Scientists have already managed to bring a plant "back from the dead"
Groundbreaking projects at the Millennium Seed Bank at Wakehurst Place in Sussex may have to be axed because of a £100m shortfall.
The money is needed over the next 10 years to maintain its work to store rare seeds from the whole plant world.
It was set up in the year 2000 with the aim of collecting all plants on earth.
Head of seed conservation department, Dr Paul Smith, said: "It works out about £2,000 per species, which is a bargain, I think, in anybody's terms."
He added: "We're talking about £100m over 10 years, to deliver a quarter of the world's plant species in safe secure storage. That's good value.
"The key thing here is the seeds that we collect and conserve and carry out research on have importance for people's livelihoods.
"We're all worried about climate change.
"We need to make sure we have those species before they disappear, so that we have options for their use."
Plant 'back from dead'
The seed bank has enough money to keep its operation running until the end of next year.
Owned and managed by the Royal Botanic Gardens at Kew, it was founded with the intention of protecting plants from extinction.
Several thousand seeds of each species are kept at the site.
Teams of scientists and technicians there are aiming to have banked 10% of the world's wild flowering plants by 2010.
They are aiming to have collected 25% by 2020, depending on whether more funding can be secured.
The Millennium Seed Bank has the capacity to store up to half the world's wild flowering plant species.
Scientists have already "brought back from the dead" a plant that has not been seen in its native habitat for more than 70 years.
Botanists helped to germinate the last remaining seeds of Belgian grass in 2005. Bromus bromoideus, once common in hayfields around Liege and Rocheford, declined rapidly from the early 1900s.