The
leaves are borne alternately on the stem. In most species, they are long,
pinnate, with (3–) 5–9 (−13) leaflets and basal
stipules; the leaflets usually have a serrated margin, and often a few small
prickles on the underside of the stem. Most roses are
deciduous but a few (particularly from
Southeast Asia) are
evergreen or nearly so.
Thorns 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
R. 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. Plant geneticist Zachary Lippman of
Cold Spring Harbor Laboratory found that prickles are controlled by the LOG gene. Blocking the LOG gene in roses reduced the thorns (large prickles) into tiny buds.
Flower The
flowers of most species have five petals, with the exception of
Rosa omeiensis and
Rosa sericea, which usually have only four. Each
petal is divided into two distinct lobes and is usually white or pink, though in a few species yellow or red. Beneath the petals are five
sepals (or in the case of some
R. omeiensis and
R. sericea, four). These may be long enough to be visible when viewed from above and appear as green points alternating with the rounded petals. The coloured petals are fused on the axis and arranged in five bundles forming a circle, the petal bundles expand further from each other; the petals form a cup or disc surrounding the
gynoecium.
Reproduction Roses are insect-pollinated in nature. A fertilized ovary forms a berry-like
aggregate fruit called a "
hip". The hips of most species are red, but a few (e.g.
Rosa pimpinellifolia) have dark purple to 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 (
R. 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. Many of the domestic
cultivars do not produce hips, as the
flowers are too tightly petalled to provide access for
pollination and the plants can only propagate through human-made
cuttings.
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.
Genome A study of the patterns of
natural selection in the
genome of roses indicated that
genes related to
DNA damage repair and stress adaptation have been positively selected, likely during their domestication. This rapid evolution may reflect an adaptation to genome confliction resulting from frequent intra- and inter-species hybridization and switching environmental conditions of growth. •
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. •
Carolinae – white, pink, and bright pink flowered species all from North America. •
Chinensis – white, pink, yellow, red and mixed-colour roses from China and
Burma. •
Gallicanae – pink to crimson and striped flowered roses from western Asia and Europe. •
Gymnocarpae – one species in western North America (
Rosa gymnocarpa), others in east Asia. •
Laevigatae – a single white flowered species from China. •
Pimpinellifoliae – white, pink, bright yellow, mauve and striped roses from Asia and Europe. •
Rosa (syn. sect.
Cinnamomeae) – white, pink, lilac, mulberry and red roses from everywhere but North Africa. •
Synstylae – white, pink, and crimson flowered roses from all areas. == Ecology ==