Orchids are grown throughout the world, being found on every continent except Antarctica. They are the largest and most successful plant group on earth. Estimates of their numbers range from 20,000 to more than 35,000 different species in 750-850 genera. Together they comprise about ten percent of the world's flowering plants. The widespread cultivation of orchids in the United States began in the early 1900's, as gardners in Southern California discovered that cymbidium orchids would do well outdoors when grown under the shade of oak trees. By the 1920's and 30's cymbidiums had become highly prized for their tall spikes filled with sprays of beautiful blossoms. As demand continued to increase, commercial nurseries started to grow cymbidiums. During World War II, English and European orchid growers sent stock to the United States to guarantee its preservation. From the 1950's on, orchid nurseries continued to thrive, making California the world center of cymbidium culture.