ROSACEAE  

Rosaceae (the rose family) is a medium-sized family of flowering plants, including about 2830 species in 95 genera.^[1] The name is derived from the type genus Rosa. Among the most species-rich genera are Alchemilla (270), Sorbus (260), Crataegus (260), Cotoneaster (260), Rubus (250),^[1] and Prunus (plums, cherries, peaches, apricots, and almonds) with about 200 species.^[2] However, all of these numbers should be seen as estimates - much taxonomic work remains.

Rosaceae includes herbs, shrubs and trees. Most species are deciduous, but some are evergreen.^[3] They have a worldwide range, but are most diverse in the Northern Hemisphere.

Several economically important products come from the Rosaceae, including many edible fruits (such as apples, pears, quinces, apricots, plums, cherries, peaches, raspberries, loquats, and strawberries), almonds, and ornamental trees and shrubs (such as roses, meadowsweets, photinias, firethorns, rowans, and hawthorns).^[3]

Distribution[edit]

The Rosaceae have a cosmopolitan distribution (found nearly everywhere except for Antarctica), but are primarily concentrated in the Northern Hemisphere in regions that are not desert or tropical rainforest.^[1]

Taxonomy[edit]

The family was traditionally divided into six subfamilies: Rosoideae, Spiraeoideae, Maloideae (Pomoideae), Amygdaloideae (Prunoideae), Neuradoideae, and Chrysobalanoideae, and most of these were treated as families by various authors.^[4] More recently, Chrysobalanoideae has also been treated as a family, but also in Rosales,^[5] but is placed in Malpighiales in molecular analyses. Neuradoideae has been assigned to Malvales. Schulze-Menz, in Engler's Syllabus edited by Melchior (1964) recognized Rosoideae, Dryadoideae, Lyonothamnoideae, Spireoideae, Amygdaloideae, and Maloideae.^[6] They were primarily diagnosed by the structure of the fruits. More recent work has identified that not all of these groups were monophyletic. Hutchinson (1964) and Kalkmann (2004) recognized only tribes (17 and 21, respectively). Takhtajan (1997) delimited 10 subfamilies: Filipenduloideae, Rosoideae, Ruboideae, Potentilloideae, Coleogynoideae, Kerroideae, Amygdaloideae (Prunoideae), Spireoideae, Maloideae (Pyroideae), Dichotomanthoideae, and 21 tribes. A more modern model comprises three subfamilies, one of which (Rosoideae) has largely remained the same. A cladogram of the family^[6] is:

Three cladistic analyses were done in 1999 by Rodger Evans, one based on the phenotype, one on molecules, and the 3rd combined.^[7] The only major difference in the results with the above cladogram is the position of Kerria, which is basal in Evans and embedded in Spireoideae in Potter et al.

While the boundaries of Rosaceae are not disputed, there is not general agreement as to how many genera into which it should be divided. 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.

Characteristics[edit]

Rosaceae can be trees, shrubs, or herbaceous plants. The herbs are mostly perennials, but some annuals also exist.^[8]

Leaves[edit]

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 Amygdaloideae (previously called Spiraeoideae).^[6] The stipules are sometimes adnate (attached surface to surface)^[9] to the petiole. Glands or extrafloral nectaries may be present on leaf margins or petioles. Spines may be present on the midrib of leaflets and the rachis of compound leaves.

Flowers[edit]

Flowers of plants in the rose family are generally described as "showy".^[10] 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 a hypanthium. They can be arranged in racemes, spikes, or heads; solitary flowers are rare.

Fruits and seeds[edit]

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, but their seeds often contain amygdalin, which can be converted to cyanide during digestion.^[11]

Genera[edit]

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 five or six tribes and 20 or more genera, including rose, Rubus (blackberry, raspberry), Fragaria (strawberry), Potentilla, and Geum.
• Subfamily Amygdaloideae: Within this group remains an identified clade with a pome fruit, traditionally known as subfamily Maloideae (or Pyroideae) which included genera such as apple, Cotoneaster, Crataegus (hawthorn), etc. 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 Amygdaloideae.^[6] 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, and includes five genera (Dryas, Cercocarpus, Chamaebatia, Cowania, and Purshia), most species of which form root nodules which host the nitrogen-fixing bacteria Frankia spp.

Economic importance[edit]

The rose family is probably one of the six most economically important crop plant families,^[12] 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.

Many genera are also highly valued ornamental shrubs; these include Cotoneaster, Crataegus, Kerria, Photinia, Potentilla, Prunus, Pyracantha, Rhodotypos, Rosa, Sorbus, Spiraea, and others.^[3]

However, 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.^[3]

References[edit]