Rosaceae (the rose family) are a medium-sized family of flowering plants, including about 2830 species in 95 genera. The name is derived from the type genus Rosa. Among the largest genera are Alchemilla (270), Sorbus (260), Crataegus (260), Cotoneaster (260), and Rubus (250). The largest genus by far is Prunus (plums, cherries, peaches, apricots and almonds) with about 430 species. However, all of these numbers should be seen as underestimates - much taxonomic work is left to be done here.
Roses can be herbs, shrubs or trees. Most species are deciduous, but some are evergreen. They have a worldwide range, but are most diverse in the northern hemisphere.
Several economically important products come from Rosaceae, including many edible fruits (such as apples, apricots, plums, cherries, peaches, pears, raspberries, and strawberries), almonds, and ornamental trees and shrubs (such as roses, meadowsweets, photinias, firethorns, rowans, and hawthorns).
The family was traditionally divided into four subfamilies: Rosoideae, Spiraeoideae, Maloideae, and Amygdaloideae, primarily diagnosed by the structure of the fruits. More recent work has identified that not all of these groups were monophyletic. A more modern view comprises three subfamilies, one of which (Rosoideae) has largely remained the same. A cladogram of the family is shown below:
While the boundaries of Rosaceae are not disputed, there is not general agreement as to how many genera it should be divided into. Areas of divergent opinion include the treatment of Potentilla s.l. and Sorbus s.l.. Compounding the problem is the fact that apomixis is common in several genera. This results in an uncertainty in the number of species contained in each of these genera, due to the difficulty of dividing apomictic complexes into species. For example, Cotoneaster contains between 70 and 300 species, Rosa around 100 (including the taxonomically complex dog roses), Sorbus 100 to 200 species, Crataegus between 200 and 1,000, Alchemilla contains around 300 species, Potentilla roughly 500, and Rubus hundreds, or possibly even thousands of species.
The leaves are generally arranged spirally, but have an opposite arrangement in some species. They can be simple or pinnately compound (either odd- or even-pinnate). Compound leaves appear in around 30 genera. The leaf margin is most often serrate. Paired stipules are generally present, and are a primitive feature within the family, independently lost in many groups of Spiraeoideae. The stipules are sometimes adnate to the petiole. Glands or extrafloral nectaries may be present on leaf margin or petiole. Spines may be present on the midrib of leaflets and the rachis of compound leaves.
Flowers of plants in the rose family are generally described as "showy". They are actinomorphic (i.e. radially symmetrical) and almost always hermaphroditic. Rosaceae generally have five sepals, five petals and many spirally arranged stamens. The bases of the sepals, petals, and stamens are fused together to form a characteristic cup-like structure called hypanthium. They can be arranged in racemes, spikes, or heads; solitary flowers are rare.
Fruits and seeds
The fruits come in many varieties and were once considered the main characters for the definition of subfamilies amongst Rosaceae, giving rise to a fundamentally artificial subdivision. They can be follicles, capsules, nuts, achenes, drupes (Prunus) and accessory fruits, like the pome of an apple, or the hip of a rose. Many fruits of the family are edible.
Identified clades include:
- Subfamily Rosoideae: Traditionally composed of those genera bearing aggregate fruits that are made up of small achenes or drupelets, and often the fleshy part of the fruit (e.g. Strawberry) is the receptacle or the stalk bearing the carpels. The circumscription is now narrowed (excluding, for example, the Dryadoideae), but it still remains a diverse group containing 5 or 6 tribes and 20 or more genera, including Rose, blackberry, raspberry, strawberry, Potentilla, Geum.
- Subfamily Maloideae (or Pomoideae): Traditionally this included only those genera (apple, cotoneaster, hawthorn, etc.) whose fruits consist of five capsules (called "cores") in a fleshy or stony endocarp, surrounded by fleshy mesocarp and hypanthium tissue. This fruit is called a pome. While this group remains an identified clade, to separate it at the subfamily level would leave the remaining genera as a paraphyletic group, so it has been expanded to include the former Spiraeoideae and Prunoideae. The subfamily has sometimes been referred to by the name "Spiraeoideae", but this is not permitted by the International Code of Nomenclature for algae, fungi, and plants.
- Subfamily Dryadoideae: Fruits are achenes with hairy styles. Includes five genera (Dryas, Cercocarpus, Chamaebatia, Cowania and Purshia), most species of which form root nodules which host the nitrogen-fixing bacterium Frankia.
The rose family is probably one of the six most economically important crop plant families, and includes apples, pears, quinces, medlars, loquats, almonds, peaches, apricots, plums, cherries, strawberries, raspberries, sloes, and roses among the crop plants belonging to the family.
On the other hand, several genera are also introduced noxious weeds in some parts of the world, costing money to be controlled. These invasive plants can have negative impacts on the diversity of local ecosystems once established. Such naturalised pests include Acaena, Cotoneaster, Crataegus, Pyracantha, and Rosa.
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