black hips. Each hip comprises an outer fleshy layer, the hypanthium, which contains 5–160 "seeds" (technically dry single-seeded fruits called achenes) embedded in a matrix of fine, but stiff, hairs. Rose hips of some species, especially the dog rose (Rosa canina) and rugosa rose (Rosa rugosa), are very rich in vitamin C, among the richest sources of any plant. The hips are eaten by fruit-eating birds such as thrushes and waxwings, which then disperse the seeds in their droppings. Some birds, particularly finches, also eat the seeds. The sharp growths along a rose stem, though commonly called "thorns", are technically prickles, outgrowths of the epidermis (the outer layer of tissue of the stem), unlike true thorns, which are modified stems. Rose prickles are typically sickle-shaped hooks, which aid the rose in hanging onto other vegetation when growing over it. Some species such as Rosa rugosa and Rosa pimpinellifolia have densely packed straight prickles, probably an adaptation to reduce browsing by animals, but also possibly an adaptation to trap wind-blown sand and so reduce erosion and protect their roots (both of these species grow naturally on coastal sand dunes). Despite the presence of prickles, roses are frequently browsed by deer. A few species of roses have only vestigial prickles that have no points. Evolution The oldest remains of roses are from the Late Eocene Florissant Formation of Colorado. Roses were present in Europe by the early Oligocene. Today's garden roses come from 18th-century China. Among the old Chinese garden roses, the Old Blush group is the most primitive, while newer groups are the most diverse. Species The genus Rosa is composed of 140–180 species and divided into four subgenera: Hulthemia (formerly Simplicifoliae, meaning "with single leaves") containing two species from southwest Asia, Rosa persica and Rosa berberifolia, which are the only roses without compound leaves or stipules. Hesperrhodos (from the Greek for "western rose") contains Rosa minutifolia and Rosa stellata, from North America. Platyrhodon (from the Greek for "flaky rose", referring to flaky bark) with one species from east Asia, Rosa roxburghii (also known as the chestnut rose). Rosa (the type subgenus, sometimes incorrectly called Eurosa) containing all the other roses. This subgenus is subdivided into 11 sections. Banksianae – white and yellow flowered roses from China. Bracteatae – three species, two from China and one from India. Caninae – pink and white flowered species from Asia, Europe and North Africa.