Prunus cerasus (sour cherry) in bloom
Prunus is an economically important genus of deciduous and evergreen trees and shrubs, characterized by a fruit in the form of a drupe, usually white to pink perigynous flowers with five petals and five sepals, simple leaves, and often the production of cyangogenetic glycosides in the seeds and leaves. There are around 430 species in this genus, mainly spread throughout the northern temperate regions of the globe. This genus includes the plums, cherries, peaches, apricots, and almonds.
The Prunus genus includes species highly valued for their fruit, as ornamentals, and for timber. The same adaptations that advance the various species own needs for reproduction and survival offer larger ecological, culinary, nutritional, commercial, and aesthetic values. Many of the fruits are readily eaten by numerous birds and mammals, which digest the fruit flesh and disperse the seeds in their droppings. These fruits also are nutritious and very popular food for human consumption, being eaten fresh or cooked. Even the bitter almond, which has toxic amounts of hydrogen cyanide, useful for repelling herbivores, can have its toxicity removed by roasting, and thus a uniquely human way of food preparation makes these fruits and their unique flavor available for people. The flowers, which are useful for attracting pollinating insects, provide the trees with great beauty and an attractive aroma, and make them popular ornamental trees. And some trees, such as the black cherry, P. serotina, are useful for providing wood for construction.
Many of the fruits have important health benefits for people, such as cherries, which are a source of vitamin C, dietary fiber, and anthocyanins, and almonds, which offer a rich source of protein, many vitamins, calcium, magnesium, potassium, among other nutrients.
The Prunus genus traditionally is placed within the rose family Rosaceae as part of the subfamily Prunoideae (or Amygdaloideae), where it is the largest genus. Prunoideae also includes one extant species of Oemleria, five species of Maderia, 90 species of Pygeus, and three to four species of Prinsepia (Bortiri et al. 2001). However, it sometimes is placed in its own family, Prunaceae (or Amygdalaceae).
The Prunoideae (Amygdaloideae) are characterized by have the fruit type as drupes and by a basic chromosome number of x = 9 (Bortiri et al. 2001). A drupe is a fruit in which an outer fleshy part (exocarp, or skin, and mesocarp, or flesh) surrounds a shell (the pit or stone) of hardened endocarp with a seed inside. These fruits develop from a single carpel, and mostly from flowers with superior ovaries. The definitive characteristic of a drupe is that the hard, lignified stone (or pit) is derived from the ovary wall of the flower. Other fleshy fruits may have a stony enclosure that comes from the seed coat surrounding the seed. These fruits are not drupes.
The flowers of members of the Prunus genus usually are white to pink, with five petals and five sepals. They are borne singly, or in umbels of two to six or sometimes more on racemes. The flowers are perigynous, meaning a half-inferior ovary (also known as “half-superior,” “subinferior,” or “partially inferior,”) is embedded or surrounded by the receptacle. In some classifications, half-inferior ovaries are not recognized and are instead grouped with either the superior or inferior ovaries. A half-inferior ovary has nearly equal portions of ovary above and below the insertion point.
A single two-ovuled carpel becomes a drupe (a "prune") with a relatively large hard coated seed (a "stone"). Leaves are simple and usually lanceolate, unlobed, and toothed along the margin.
Many Prunus species produce hydrogen cyanide, usually in their leaves and seeds. This gives a characteristic taste in small (trace) quantities, and becomes bitter in larger quantities. The cyanogenetic glycosides found in Prunus species are amygdalin, prulaurasin, and prunasin. This makes some Prunus species toxic, although the fruit usually is safe. Bitter almonds, produced from Prunus amygdalus var. amara can be dangerous if eaten raw because they yield significant amounts of prussic acid (hydrogen cyanide), from the enzyme emulsin acting on a soluble glucoside, amygdalin. They must be roasted to overcome the toxicity. Likewise, the seeds or kernals of the pits of apricots (Prunus armeniaca) are poisonous until roasted. The leaves of chokecherries, such as P. virginiana and P. virginiana var. melanocarpa) can be poisonous to horses that may eat the leaves.
The earliest fossil Prunus are wood, drupe, seed, and a leaf from the middle Eocene of the Princeton Chert of British Columbia (Stockey and Wehr 1996). Using the known age as calibration data, recent research by Oh and Potter (2005) reconstructs a partial phylogeny of some Rosaceae from a number of nucleotide sequences. According to this study, Prunus and its "sister clade" Maloideae (apple subfamily) diverged at 44.3 mya (well before most of the Primates existed). This date is within the Lutetian, or older middle Eocene.
In 1737 Linnaeus used four genera to include the species of modern Prunus: Amygdalus, Cerasus, Prunus and Padus. However, in 1754, Linneaus simplified this to Amygdalus and Prunus (Bortiri et al. 2001). Since then the various genera of Linnaeus and others have become subgenera and sections, as it clearer that all the species are more closely related. Bailey (1898, 181) notes, "The numerous forms grade into each other so imperceptibly and inextricably that the genus cannot be readily broken up into species."
A recent DNA study of 48 species concluded that Prunus is monophyletic and is descended from some Eurasian ancestor (Bortiri et al. 2001).
While historical treatments break the genus up into several different genera, such as segregation is not currently widely recognized other than at the subgeneric rank. ITIS (1999) recognizes just the single genus Prunus with the various species and no listed subgenera.
One standard contemporaneous treatment of subgenera derives from the work of Alfred Rehder in 1940. Rehder hypothesized five subgenera: Amygdalus, Prunus, Cerasus, Padus and Laurocerasus (Lee and Wen 2001). To these subgenera, Ingram added Lithocerasus (Okie 2003). The six contemporaneous subgenera are described as follows:
Another recent DNA study (Lee and Wen 2001) found that Amygdaloideae can be divided into two clades: Prunus-Maddenia, with Maddenia basal within Prunus, and Exochorda-Oemleria-Prinsepia. Prunus can be divided into two clades as well: Amygdalus-Prunus and Cerasus-Laurocerasus-Padus. Yet another study adds Empectocladus as a subgenus to the former (Bortiri et al. 2002).
The genus Prunus includes a number of important cultivated species, including the almond, apricot, cherry, peach, and plum, all of which have cultivars developed for commercial fruit and "nut" production. The edible part of the almond is the seed; the almond seed is a drupe and not a true "nut."
There are also a number of species, hybrids, and cultivars grown as ornamental plants, usually for their profusion of flowers, sometimes for ornamental foliage and shape, occasionally for their bark. These ornamentals include the group that may be collectively called flowering cherries (including sakura, the Japanese flowering cherries).
Other species such as blackthorn are grown for hedging, game cover, and other utilitarian purposes.
The wood of some species is a minor and specialized timber (cherry wood), usually from larger tree species such as the wild cherry or black cherry.
Many species produce an aromatic resin from wounds in the trunk; this is sometimes used medicinally. There are other minor uses, including dye production.
Pygeum is a herbal remedy containing extracts from the bark of Prunus africana. It is used as to alleviate some of the discomfort caused by inflammation in patients suffering from benign prostatic hyperplasia.
Because of their considerable value as both food and ornamental plants, many Prunus species have been introduced to parts of the world to which they are not native, some becoming naturalized.
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