Bamboo is a group of woody perennial plants in the true grass family Poaceae. In the tribe Bambuseae also known as bamboo, there are 91 genera and over 1,000 species. The size of bamboo varies from small annuals to giant timber bamboo. Bamboo evolved only 30 to 40 million years ago, after the demise of the dinosaurs. Bamboo species can be divided into clumping (sympodial) and running (monopodial) species. Clumping species grow from the soil in a slowly expanding tuft. Running species send underground rhizomes to produce shoots several metres from the parent plant.