From an underground "forest" to spectacular orchids, scientists at the Royal Botanic Gardens, Kew, discovered 74 new plants and 15 fungi last year.
Many of the mysterious species were found in unlikely places, such as on the top of a volcano or clinging to Antarctic rocks.
The new finds need immediate protection and at least one will probably already have been lost, the scientists say.
About three-quarters of undescribed plants are threatened with extinction.
Three new species of Antarctic fungi
Antarctica is virtually devoid of flowering plants - but lichens can gain a foothold on small areas of exposed bare rock.
Only 5-10% of all existing fungal species are known - found not just in remote unexplored areas but everywhere on the planet.
An orchid found at the top of a volcano
The plant, with spectacular bright red flowers, was found living on the summit of Mount Nok - an extinct volcano on the Indonesian island of Waigeo.
A palm that grows underground
The palm, named Pinanga subterranea, was found on the island of Borneo, in South East Asia.
The bright red fruits and flowers are largely buried underground.
