Paris - Botanical gardens are a Noah's Ark for endangered plants, collectively safeguarding four out of 10 species known to face extinction, researchers said on Monday.But the otherwise reassuring number hides a serious imbalance, they reported in the science journal Nature Plants. Such gardens, which often double as research centres, house only a quarter of tropical flora, as opposed to 60% of plants native to temperate climes.Species diversity is far richer in the tropics, but more than 90% of vegetal safe havens are in the northern hemisphere.Plants are essential for life on Earth. They play a crucial role in regulating the climate, maintaining soil fertility, and the purification of water and air. They also provide food, medicines, building materials and fuel to sustain human life.And yet more than a fifth of wild plant diversity is threatened with extinction, earlier research has shown.The biggest threat is humanity's expanding "footprint", already encroaching on three-quarters of all land area - 40% of which has been given over to agriculture."The global network of botanical gardens is our best hope for saving some of the world's most endangered plants," said senior author Samuel Brockington, a researcher at the University of Cambridge in England. "If we fail to conserve our plant diversity, humanity will struggle to solve the global challenges of food and fuel security, environmental degradation, and climate change."Understanding evolutionTo find out what percentage of plant species are housed in captivity, Brockington and colleagues compared databases cataloguing all known flora to those held by a third of the world's 3 269 botanical gardens.More than a third of Earth's 350 699 known plant species are to be found in at least one such institution, the scientists reported.Botanical gardens also house more than 40% of the nearly 32 000 plant species classified as threatened on the Botanic Gardens Conservation International (BGCI) ThreatSearch database.It is by far the world's most comprehensive list of threatened flora.The researchers showed that only 10% of the area in botanical gardens is given over to these endangered plants.They also revealed that Earth's most ancient living flora, so-called non-vascular plants such as mosses and some algae, have been overlooked and neglected, comprising less than five percent of species under care."Non-vascular species are the living representations of the first plants to colonise land," Brockington said. "They are essential for understanding the evolution of plants."Scientists have so far assessed the threat status of about 26% of the world's known plant species.