The project brought together scores of experts at RBG's famous Kew Gardens in London with the Missouri Botanical Garden in St. Louis.

It traces its origins to a 1999 botanical congress which called for a clear picture of plant biodiversity to help preserve species under threat.

The Plant List is described as a working list that will require finetuning.

"(It) is really a major step forward," said Peter Wyse Jackson, president of the Missouri Botanical Garden.

"It provides for the first time a basic checklist of what plants there are on the planet, and it can be used for so many purposes, planning conservation, action looking at the economic importance of plants and so on."

Of the 1.25 million names, 1.04 million are of species rank while the remainder are "infraspecific," meaning they are families or sub-groups of species.

The longest name is Ornithogalum adseptentrionesvergentulum, for a group of species that includes the ?Star of Bethlehem? plant. The shortest names include Poa fax, or scaly poa, a purplish flower native to Western Australia.

Only 300,000 names for species have been accepted as standard terms by the experts, and 480,000 others have been deemed "synonyms," or alternatives to accepted names.

A whopping 260,000 names are "unresolved," meaning that data is too sketchy to determine swiftly whether the claim for a new plant find is backed by the facts. This part of the list will be whittled down by experts over the years to come.

Under a plan adopted in Nagoya, Japan, last October, members of the UN's Biodiversity Convention agreed to set up a complete plant database by 2020.

One in five of the world's known plant species is under threat of extinction, the International Union for Conservation of Nature (IUCN) said in September.

Habitat loss, climate change, pollution and invasive species are the major perils.