Rose Family (Rosaceae)
The rose family (Rosaceae), in the order Rosales, is a large plant family containing more than 100 genera and 2,000 species of trees, shrubs, and herbs. This family is represented on all continents except Antarctica, but the majority of species are found in Europe, Asia, and North America. Fossil evidence from Colorado, reliably identified as belonging to the genus Rosa, suggests that this family has been in existence for at least 35 million years.
Most species in the Rosaceae have leaves with serrated margins and a pair of stipules where the leaf joins the stem. The majority of tree-sized arborescent species have leaves that are simple except for species of mountain ash (Sorbus spp.), which have compound leaves divided into five to seven leaflets. Conversely, most woody shrubs and herbs have compound leaves which are composed of three to 11 leaflets. Branch spines and prickles are common on trees and shrubs in the rose family. However, there is variability in the appearance of these structures even among species which occur in very similar habitats. For example, blackbrush, (Coleogyne ramosissima), a species found in pinion-juniper woodlands in the American Southwest, has long spines on which it bears flowers, while Apache plume (Fallugia paradoxa, is found in the same region and habitat but has no spines. On a much larger scale, trees of the genus Crataegus, which are collectively called thornapples or hawthorns, have prominent branch spines while most species of Malus and Prunus are without spines. Herbaceous species typically lack spines or prickles.
Flowers in this family are typically radially symmetrical flat discs (actinomorphic) and contain both male and female floral structures in a single flower. Flower ovaries may be positioned below the sepals and petals (inferior) or above them (superior). In flowers having an inferior ovary, the carpels are surrounded by a hollow receptacle. Flowers typically have five sepals, five petals, numerous stamens, and one to 50 carpels. Carpels in this family tend to remain free instead of becoming fused into a many chambered, single carpel. Anthers have two chambers, called locules, which split lengthwise to release thousands of pollen grains. Another distinguishing feature of flowers in this family is the presence of a structure called the epicalyx. The epicalyx is composed of five sepal-like structures which occur below and alternate with the true calyx.
Most species have large white, pink, or red petals which are designed to attract pollinating insects. Many white and pale pink flowers also produce volatile esters, chemicals which we perceive as pleasant odors, but are produced to attract insects. The chief pollinators of rose flowers are bees ranging in size from tiny, metallic green flower bees of the genus Augochlora, through honey bees (Apis), to large bumble bees (Bombus). These pollinators are unspecialized and also pollinate many other species which have actinomorphic flowers and offer copious pollen as a reward for flower visitation.
Insect pollination is the most common type in the Rosaceae, but some species have evolved to be pollinated by wind. Flowers adapted for wind pollination are found in species of Acaena, which are native to windswept mountain areas of New Zealand, Australia, and the Andes Mountains of South America. Wind pollination also occurs in species of the genus Poterium which are native to high elevations in Europe, western Asia and northern Africa. Both of these genera inhabit habitats where the combination of frequent low temperatures and windy periods make wind more reliable than insects as a mechanism to achieve pollination. Also, unlike the usual bisexual condition found in insect pollinated flowers, species of Acaena and Poterium have distinct male and female flowers.
Woody shrubs and herbs in the Rosaceae also propagate through asexual means. Shrubs in the genera Chaenomeles (flowering quince), Rosa (Rose), and Rubus (blackberry and raspberry) produce suckers from their rootstock or spread by rhizomes. Species of Rubus may also spread by stems that produce roots when they bend and the tip touches the ground. Some herbaceous species of the genera Fragaria (strawberry), Duchesnea (Indian strawberry), and Potentilla (cinquefoil) produce plantlets at the end of stolons that take root and eventually live as independent, but genetically identical plants.
There are many different types of fruits in the rose family, ranging from single-seeded, soft, fleshy, fruits known as drupes to harder, fleshy pseudocarps such as a pome or hip. In the genera Malus (apples and crabapples) , Chaenomeles, and Rosa, the true fruit is engulfed in a fleshy structure called the hypanthium, which is composed of the swollen bases of petals and sepals. In the mature pseudocarp (pome or hip), the true fruit is centrally located and contains five distinct carpels which may contain one or more seeds each. The fleshy tissue which surrounds the fruit is the hypanthium. This type of fruit is called a pome or hip.
In the genus Prunus (cherry, peach, and plum), fruits contain a single seed enclosed in a hard structure that is not part of the seed coat called the endocarp. The mesocarp and ectocarp are fleshy. This type of fruit is called a drupe.
Other members of the rose family have a small drupe called a drupelet, as in the genus Rubus. In these plants, several distinct pistils are attached to the receptacle, each of which becomes a drupelet. Because there are as many as 30 drupelets on each receptacle, the fruit of a blackberry is referred to as a aggregate fruit. The commercial raspberry is the result of crosses among the dominant parent plant, Rubus ideas, and other Rubus species. Similarly, in the genus Fragaria (strawberry), there are as many as 50 distinct, single-ovule pistils in each individual flower. Here, however, the matured carpel becomes a small, dry, hard, and single-seed containing fruit called an achene. The bright red structure on which all these achenes rest is developed from the floral receptacle and is the part of the flower which we eat. The commercial strawberry is a cultivated version of the sand strawberry, Fragaria chiloensis, which is native to dunes on the western coast of North America.
Most members of the Rosaceae have fruits that are fleshy and conspicuously red, purple or yellow in color. These fruits serve as important sources of nutrition for many species of wild animals. From the evolutionary perspective of the plant, the function of these edible fruits is not primarily to serve as food. Instead, these pomes, drupes, and aggregate fruits are designed to entice an animal into eating the fruit, so the enclosed seeds are then either discarded or ingested. In this way, the plant offers food to the animal, and the animal acts as an agent of seed dispersal for the plant. The hard endocarp of drupes and drupelets enables the enclosed seed to pass safely through the digestive tract of a bird and to be excreted intact.
Certain species in the Rosaceae are also of importance because of their value as ecological indicators of habitat conditions. In open habitats where soil is acidic, species such as the cinquefoils, Potentilla canadensis and P. simplex, can become common understory herbs. Also in this type of habitat, Indian strawberry (Duchesnea indica) may become quite common. Duchesnea indica is interesting because it has a similar appearance and growth habit to strawberries (Fragaria). However, where true strawberries have flowers with white petals, D. indica has yellow petals. Also, leaflets of Fragaria species have smaller serrations on the margins, and are more generally round in shape than are leaflets of D. indica. The rose family has both specialized and unspecialized insect herbivores. Unspecialized herbivores such as the rose chafer, (Macrodactylus subspinosis), and the Japanese beetle, (Popillia japonica), eat the flowers of roses and other plants. More specialized herbivores include the rose curculio, (Rhynictes bicolor), a bright-red weevil that eats parts of flowers in the rose genus (Rosa) and is rarely found on flowers of other genera in the Rosaceae, or on species of other plant families. One of the most specialized herbivores is the rose leafhopper, (Typhlocyba rosae), which has adjusted to the secondary chemistry of rose plants and does not attack flowers, but instead feeds on sap from stems.
Most of the tree-sized species of the Rosaceae which provide us with edible fruit, such as apricot (Prunus armeniaca), domestic apple (Malus pumila), peach (Prunus persica), pear (Pyrus communis and P. pyrifolia), and plum (Prunus domestica), are native to Europe and Asia and have been in cultivation for hundreds of years. Today, there are relatively few cultivars of apple, peach, and plum available for sale. However, 100 years ago there were many different cultivated versions of each of these species. One of the most popular cherry trees in cultivation, sour cherry (Prunus cerasus), is also probably native to Europe or Asia, although its true origin is unknown.
In addition, many species of Malus and Prunus are native to North America. Beach plum (Prunus maritima) and black cherry (P. serotina) are common members of barrier island maritime forest and mainland forests of southeastern North America. Choke cherry (Prunus virginiana) and sweet cherry (Prunus avium) are common components of recently disturbed areas within inland forests in eastern North America. Sweet cherry is also a popular cultivated species.
Climbing species of Rosa are far less common than those with a shrub growth habit. Species such as dog rose (R. canina) of Europe and R. virginiana of eastern North America are noted for their prodigious growth, in which stems may attain lengths of several meters. This is possible because these climbing species do not devote as much growth to structural support, as do shrub roses, and instead use surrounding vegetation for support. With this growth form, climbing roses may obscure and kill supporting vegetation and can cover a substantial surface area with an impenetrable thicket. European folk tales feature the vigorous growth of climbing rose plants which was said to have engulfed even the largest man-made structures. In fact, the stems of the dog rose may reach 9.8 ft (3 m) in length. This may have been enough engulf an abandoned cottage. This probably suggests the extent to which species such as R. canina, also called English briar, have been associated with human culture since ancient times.
The genus Rosa is of major importance in the floriculture industry, and today there are well over 300 kinds of hybrid roses in cultivation. Rose hybrids are divided into "new" and "old" types. Old hybrid roses such as "Rosa Mundi" and "Frau Karl Drushki" result from simple crosses between European species and moderate selection for double-petalled flowers. Some of the older hybrid roses retain functional anthers and may form hips.
The modern hybrids, such as "Peace," differ from old hybrids in that hybridization has been more intensive and selection has led to exclusively sterile polypetalous cultivars. Because selection has focused on obtaining forms with large flowers and many petals, modern hybrid roses are commonly not very resistant to pathogens, and are susceptible to bacterial and fungal infections. Also, where wild roses suffer few major infestations from insect herbivores, modern rose hybrids are susceptible to attack from many generalist herbivores including species of aphids and earwigs.
In addition to the genus Rosa, many other members of the rose family are also valued as ornamentals. Plants of the genera Chaenomeles, Filipendula, Geum, Kerria, Potentilla, and Spirea, are commonly used in landscaping and in flower gardens as ornamentals. Some species of Potentilla and Geum native to Europe and Asia have also been extensively hybridized to yield double petalled, sterile cultivars. One species of Geum native to North America, G. rivale, or Indian chocolate, was once a dietary item in the cultures of Native American groups in eastern North America.
Trees in the genera Crataegus, Cotoneaster, and Sorbus are valued not only for their flowers but also for their interesting leaves and fruit clusters. Another popular cultivated member of the rosaceae is the climbing, woody plant Pyracantha coccinea. This plant produces many clusters of white flowers in spring and orange-red fruits in fall which are eaten by migrating birds.
In addition to important contributions to our food and horticulture, the Rosaceae has been important in human culture. The best-known flower in the family is that of the genus after which the family is named, Rosa. This genus is well represented in Europe and the Mediterranean region, where it has been used for ornamental purposes for several thousand years. The earliest known, man-made image of a rose is in a fresco found in the city of Knossos on Crete. This image dates back to the sixteenth century B.C. On the nearby island of Rhodes, 6,000-year-old coins had the image of a rose flower. The island's name, Rhodes, may in fact be derived from the word rose.
In many cultures of Europe and Asia a white rose flower symbolizes purity, while a red rose flower symbolizes strength. In ancient Greece and Rome, rose petals were strewn along the path where important people walked, and in Sybaris, an ancient city in Italy, mattresses were filled with rose petals. This may be where we get the phrase, "a bed of roses." The Romans may have also constructed special houses for the cultivation of rose plants during the winter. These houses were heated by hot water running through pipes. This system would have made rose petals available for use during winter festivities. Also, during certain festivals, when a rose flower was placed on the ceiling of a room, anything said sub rosa, (that is, "under the rose"), could not be repeated to anyone else.
Rose flowers have also been important in British heraldry. For example, rose flowers were traditional symbols used by royal families in England. White and red roses were the symbols of the two competing royal lines of England that fought the War of the Roses. Another famous member of the rose family, the rowan tree (Sorbus aucuparia), was sacred to the Celtic peoples.
The genus Rosa is widespread and indigenous to many areas of North America, Asia, and Europe. The majority of Rosa species grow in a shrub habit, and can be difficult to tell apart at first glance. Several other species native to Europe and North America grow as climbing vines or brambles. European and Asian shrub species such as Turkestan rose (R. rugosa), damask rose (R. damascena), and tea rose (R. odorata), have been grown near human habitations for centuries, and have been extensively hybridized in horticulture. In North America, similar appearing shrub roses can be found in a wide range of habitats. Species such as swamp rose (Rosa palustris) can be found in low and marshy ground in the east, while prairie rose (R. arkansana) grows in dry upland areas of the tallgrass prairie in the midwest. In the arid southwest, Fendler rose (R. fendleri) can be found growing on dry mountain slopes, while Arizona rose (R. arizonica) can be found growing along streams and forest edges. While most of these shrub rose species may attain mature heights of 3.3-6.6 ft (1-2 m), their root systems may be far more substantial. For example, the root system of Rosa arkansana may extend to a depth of 19.7-23 ft (6-7 m) into the soil.
Because Atlantic coastal barrier islands are located along migratory bird routes and because few wind-dispersed plant seeds may reach these remote islands, the maritime forest plant communities are composed of many bird dispersed species, many of which belong to the rose family. For example, Prunus maritima and P. serotina are commonly found on barrier islands from Massachusetts to Florida. Birds eat fruits from established plants on some islands and defecate seeds onto different islands, thereby spreading these plants across most of the chain of barrier islands. This relationship, between species of Prunus and birds, is exemplified by the species named Prunus avium, also called bird cherry and by one of the common names for P. serotina, wild bird cherry. This relationship between many fruits produced by members of the rose family and birds is common, and is also the reason why certain species such as blackberry and hawthorn can often be found growing in suburban lawns when no parent plants are established in the immediate area. While most fruits of the rose family are eaten by birds, fruits of the prostrate growing strawberries may be eaten by a wider variety of wildlife such as mammals and reptiles. For instance, the aggregate fruits of wild strawberry (Fragaria virginiana) are a favorite food of box turtles (Terrapene ornata and T. caroliniana).
Many members of the rose family, particularly species of the intermountain west, are important forage plants for cattle. Species such as bitter cherry (Prunus emarginata), cliffrose (Cowania mexicana var. stansburiana), desert peach (Prunus andersonii), and fern bush, (Chamaebataria millefolium) are eaten by sheep and cattle and are browsed on by deer. Perhaps the most important species to cattle ranchers is bitterbrush (Purshia tridentata). Bitterbrush is similar in appearance to sagebrush (Artemisia tridentata; family Asteraceae), and grows in the same ecological conditions. However, while sagebrush is not edible, bitterbrush is edible, nutritious, and abundant.
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Stephen R. Johnson