Plants are a kingdom of life forms that includes familiar organisms such as trees, herbs, bushes, grasses, vines, ferns and mosses. Through photosynthesis, they convert water and carbon dioxide into the oxygen we breathe and the sugars that provide the primary fuel for life. Through nitrogen fixation, plants generate proteins that are basic building blocks of life. Early fossil records of photosynthesizing organisms date from about 3 billion years ago. Plants were instrumental to evolution as a whole in that they produced the oxygen that made life on Earth possible — not only by "breathing" it into the atmosphere and transforming it, but also by crushing rocks with their roots, which created soils and released nutrients on a large scale. Plants are crucial to the existence of all other living creatures on Earth, both through the systemic life-support services they sustain and the food, medicine and other material resources they provide. The total number of described plant species hovers around 250,000.

Our information on plant species numbers is far from perfect. The International Union for the Conservation of Nature's Red List estimates that about 54 percent of evaluated plant species (not nearly all the plants on Earth) are threatened: 10,584 out of 19,738. Other estimates say about one in eight plants is considered at risk of extinction globally, putting the U.S. number at one-third. The Red List says 279 evaluated plants are threatened in the United States.