|Prickly pears typically grow with flat, rounded cladodes (also called platyclades) that are armed with two kinds of spines; large, smooth, fixed spines and small, hairlike prickles called glochids that easily penetrate skin and detach from the plant. Many types of prickly pears grow into dense, tangled structures.
Like all true cactus species, prickly pears are native only to the Americas; however, they have been introduced to other parts of the globe. Prickly pear species are found in abundance in Mexico, especially in the central and western regions, and in the Caribbean islands (West Indies). In the United States, Prickly Pear are native to many areas of the arid Western United States, including the lower elevations of the Rocky Mountains such as inColorado, where species such as Opuntia phaeacantha, Opuntia polyacantha and others become dominant, and especially in the desert Southwest. Prickly pear cactus is also native to the dry sandhills and sand dunes of the East Coast from Florida to Connecticut/Long Island ( Opuntia humifusa ). Further north, Opuntia occurs in isolated areas from the southern Great Lakes to southern Ontario, Canada. O. humifusa is also a prominent feature of the flora at Illinois Beach State Park, in Winthrop Harbor, Illinois, north of Chicago, and of Indiana Dunes State Park southeast of Chicago.
In the Galapagos Islands, there are six different species: O. echios, O. galapageia, O. helleri, O. insularis, O. saxicola, O. megasperma. These species are divided into 14 different varieties; most of these are confined to one or a few islands. For this reason, they have been described as "an excellent example of adaptive radiation". On the whole, on islands where there are tall, trunked varieties there are also giant tortoises, and islands lacking tortoises have low or prostrate forms of Opuntia.
The first introduction of prickly-pear into Australia can be definitely ascribed to Governor Philip and the earliest colonists in the year 1788. Brought from Brazil to Sydney, they remained in Sydney, New South Wales, where it was rediscovered in a farmer's garden in 1839. The farmer's wife gave cuttings to neighbors and friends, who planted it not only in their gardens but also as hedgerows. So began the Australian invasion that caused major ecological damage in the eastern states. They are also found in the Mediterranean region of Northern Africa, especially in the most northern nation of Africa, Tunisia, where they grow all over the countryside, and southern Europe, especially on the island nation of Malta, where they grow all over the islands, in the south-east of Spain, and can be found in enormous numbers in parts of South Africa, where it was introduced from South America.
Opuntia species are the most cold-tolerant of the lowland cacti, extending into western and southern Canada; one subspecies, Opuntia fragilis var.fragilis, has been found growing along the Beatton River in central British Columbia, southwest of Cecil Lake at 56° 17’ N latitude and 120° 39’ W longitude. Prickly pears also produce a fruit that is commonly eaten in Mexico, known as tuna; it also is used to make aguas frescas. The fruit can be red, wine-red, green or yellow-orange.
Charles Darwin was the first to note that these cacti have thigmotactic anthers: when the anthers are touched, they curl over, depositing their pollen. This movement can be seen by gently poking the anthers of an open Opuntia flower. The same trait has evolved convergently in other cacti.