Plant species are under threat of extinction, whether due to by human activity (smuggling, digging up plants from the wild, pollution, ever-widening urban areas) or natural causes (droughts, floods, lightening storms and fires). Although endangered species of plant are protected by international Convention, this Convention is very hard to enforce and frequently updated. Many species are lost before or within a year or two after they are described by botanists. Also, those countries in which the endangered species are grown are often not signatories of the Convention. Accidental removal of rare plants is also a problem. For example, neither the farmers uprooting plants and bulbs for sale as decorative house plants, nor the companies selling these plants in the mass market, may realize the endangered nature of the species with which they are dealing.
Plants have historically provided some of the most important drugs available to humanity, such as morphine, aspirin and quinine, as well as a number of less common drugs such as anti-cancer medications derived from the periwinkle.
Plant taxonomy is very variable and has been much influenced recently by genetic techniques. Major divisions are into "flowering plants" (angiosperms) and "seed plants" (gymnosperms). Cryptogam is a general name for plants, and plant-like organisms, that lack flowers and are not reproduced by seeds, including ferns, mosses, fungi and algae, even though some of these groups are now not recognized as plants but parallel evolutionary developments to plants.
Of the roughly 270,000 kinds of vascular plants, probably no more than 1,000 have become extinct during the past century, but the pace of extinction is increasing rapidly at present and will almost certainly continue to do so over the next few decades. About a third of the world's vascular plants occur in temperate regions; here threatened and endangered plant species are in general well understood and well monitored. Conservatively, about 10% of the species in temperate regions might be classified as threatened or endangered, the greatest number of such species being in the Cape region of southern Africa. The greatest threat to plant species, however, exists in the tropics. About 170,000 species occur in the tropics and subtropics. Especially notable is the fact that more than 40,000 plant species - a quarter of the world's total diversity - occurs in the three northern Andean countries of Colombia, Equador, and Peru combined, countries in which the plants, animals and microorganisms are the most poorly known assemblage on earth.
To be classified as threatened, a plant species must have reached the point at which there are fewer than 10,000 individuals worldwide or fewer than 100 locations in which it is found. In 1997 it was reported that habitat destruction and the introduction of non-native species has caused over 34,000 plant species worldwide to become so rare that they could easily disappear. That amounts to 12.5 percent of the fern, conifer and flowering species known worldwide. Of the endangered species, 91 percent exist in no more than one country. In the USA, 29 percent of the 16,000 species are at risk. Similar percentages were recorded for Australia and South Africa.
In some cases, entire plant families are in trouble. For example, 75 percent of the yew family—which produces the anti-cancer drug taxol–is threatened with extinction globally. Even familiar groups are in trouble, including approximately 14 percent of roses, 32 percent of lilies and irises and 29 percent of palms.
A third of tropical African plants are on the path to extinction, according to a 2019 assessment. Much of western Africa, Ethiopia, and parts of Tanzania and the Democratic Republic of the Congo are the hardest hit regions, standing to lose more than 40% of their richness of plants. Species at risk include trees, shrubs, herbs and woody vines. Threats include deforestation, population growth and climate change, the scientists said.