The Royal Botanical Gardens (RBG) Kew has published its ‘State of the World’s Plants 2016’ report, which, they say, ‘provides, for the first time, a baseline assessment of our current knowledge on the diversity of plants on earth, the global threats these plants currently face, and the policies in place and their effectiveness in dealing with these threats.’
The 80-page report quantifies an estimated ~391,000 vascular plants known to science, of which 31,128 plant species have a documented use such as medicine (17,810 medicinal plant species or 57.2% of known ‘useful’ plants), plants used for textiles and building materials (11,365 species or 36.5%), and human food (5,538 species or 17.8%), among other categories of useful plants.
Due to a range of factors, the authors estimate that about 21% of the world’s plants are currently threatened with extinction. The three biggest factors threatening plant species with extinction are the destruction of habitats, for example due to land-use change for farming and cattle ranching (31%), deforestation for timber (21.3%), and residential and commercial development (12.8%).
In conjunction with the launch of the report, scientists and policymakers will gather at Kew for the first annual ‘State of the World’s Plants Symposium’, 11-12 May 2016.

Sources:
1. Carrington D. One in five of world's plant species at risk of extinction. The Guardian. 09 May 2016: http://www.theguardian.com/environment/2016/may/10/one-in-five-of-worlds-plant-species-at-risk-of-extinction
2. RBG Kew (2016) The State of the World’s Plants Report - 2016. Royal Botanical Gardens, Kew: http://science.kew.org/strategic-output/state-worlds-plants