Adansonia
Adansonia is a genus of medium-to-large deciduous trees known as baobabs. The eight species of Adansonia are native to Africa, Australia, and Madagascar but have also been introduced to other regions of the world, including Barbados, where several of the baobabs there are suspected to have originated from Africa. Other baobabs have been introduced to Asia. A genomic and ecological analysis further suggests that the genus itself originated from Madagascar.
The generic name Adansonia honours Michel Adanson, the French naturalist and explorer who provided the first detailed botanical description and illustrations of Adansonia digitata. The baobab, however, is also known as the "upside down tree," a name attributable to the trees' overall appearance and historical myths. Baobabs are among the longest-lived of vascular plants and have large flowers that are reproductive for a maximum of 15 hours. The flowers open around dusk with sufficiently rapid movement that is detectable by the naked eye. The fruits are large, oval to round and berry-like, and hold kidney-shaped seeds in a dry, pulpy matrix.
In the early 21st century, baobabs in southern Africa began to die off rapidly and mysteriously—the cause is yet to be determined. Blight or pests are unlikely to have caused such rapid death, so some have speculated that the cause may have been mass dehydration.
Description
Baobabs are long-lived deciduous, small to large trees from tall with broad trunks and compact crowns. Young trees usually have slender, tapering trunks, often with a swollen base. Mature trees have massive trunks that are bottle-shaped or cylindrical and tapered from bottom to top. The trunk is made of fibrous wood arranged in concentric rings, although rings are not always formed annually and so cannot be used to determine the age of individual trees. Tree diameter fluctuates with rainfall so it is thought that water may be stored in the trunk. Baobab trees have two types of shoots—long, green vegetative ones, and stout, woody reproductive ones. Branches can be massive and spread out horizontally from the trunk or are ascending.Adansonia gregorii is generally the smallest of the baobabs, rarely getting to over tall and often with multiple trunks. Both A. rubrostipa and A. madagascariensis are small to large trees, from tall. The other baobabs grow from tall, with diameter trunks. A. digitata, however, often has massive single or multiple trunks of up to diameter.
Leaves
Leaves are palmately compound in mature trees, but seedlings and regenerating shoots may have simple leaves. The transition to compound leaves comes with age and may be gradual. Leaves have 5–11 leaflets, with the largest ones in the middle and may be stalkless or with short petioles. Leaflets may have toothed or smooth edges, and may be hairless or have simple-to-clumped hairs. Baobabs have stipules at the base of the leaves, but the stipules are soon shed in most species. Baobabs are deciduous, shedding leaves during the dry season.Flowers
In most Adansonia species, the flowers are borne on short erect or spreading stalks in the axils of the leaves near the tips of reproductive shoots. Only A. digitata has flowers and fruits set on long, hanging stalks. There is usually only a single flower in an axil, but sometimes flowers occur in pairs. They are large, showy and strongly scented. They only open near dusk. Opening is rapid and movement of the flower parts is fast enough to be visible. Most Adansonia species are pollinated by bats.Flowers may remain attached to the trees for several days, but the reproductive phase is very short, with pollen shed during the first night and stigmas shriveled by the morning. The flower is made up of an outer 5-lobed calyx, and an inner ring of petals set around a fused tube of stamens. The outer lobes of the calyx are usually green and in bud are joined almost to the tip. As the flower opens, the calyx lobes split apart and become coiled or bent back at the base of the flower. The inner surface of the lobes are silky-hairy and cream, pink, or red. Sometimes the lobes do not separate cleanly, distorting the shape of the flower as they bend back. The calyx lobes remain fused at the base, leaving a feature that has nectar-producing tissue and that is cup-shaped, flat or tubular; the form of the calyx tube varies with species. The flowers have a central tube made up of fused stalks of stamens, with unfused filaments above. A densely hairy ovary is enclosed in the staminal tube, and a long style tipped with a stigma emerges from the filaments. Petals are set near the base of the staminal tube and are variable in shape and colour. The flowers, when fresh, may be white, cream, bright yellow or dark red, but fade quickly, often turning reddish when dried.
Fruit
The fruit of the baobabs is one of their distinguishing features. It is large, oval-to-round, and berry-like in most species. It has a dry, hard outer shell of variable thickness. In most species, the shell is indehiscent. A. gibbosa is the only species with fruits that crack while still on the tree, which then tend to break open upon landing on the ground. Inside the outer shell, kidney-shaped seeds 10–15 mm long are set in a dry pulp.Taxonomy
The earliest written reports of baobab are from a 14th-century travelogue by the Arab traveler Ibn Battuta. The first botanical description was in the De medicina Aegyptiorum by Prospero Alpini, looking at fruits that he observed in Egypt from an unknown source. They were called Bahobab, possibly from the Arabic أَبُو حِبَاب abū ḥibāb meaning "many-seeded fruit". The French explorer and botanist Michel Adanson observed a baobab tree in 1749 on the island of Sor in Senegal, and wrote the first detailed botanical description of the full tree, accompanied with illustrations. Recognizing the connection to the fruit described by Alpini he called the genus Baobab. Linnaeus later renamed the genus Adansonia, to honour Adanson, but use of baobab as one of the common names has persisted.The genus Adansonia is in the subfamily Bombacoideae, within the family Malvaceae in the order Malvales. The subfamily Bombacoideae was previously treated as the Bombacaceae family but it is no longer recognized at the rank of family by the Angiosperm Phylogeny Group I 1998, II 2003 or the Kubitzki system 2003. The closest living relatives of Adansonia within Bombacoideae are the South American genera: Cavanillesia, Scleronema, Catostemma, and Aguiaria. There are eight accepted species of Adansonia. A new species, was described in 2012, found in high-elevation sites in eastern and southern Africa. This, however, is no longer recognized as a distinct species but considered a synonym of A. digitata. Some high-elevation trees in Tanzania show different genetics and morphology, but further study is needed to determine if recognition of them as a separate species is warranted. The genus Adansonia is further divided into three sections. Section Adansonia includes only A. digitata. This species has hanging flowers and fruit, set on long flowering stalks. This is the type species for the genus Adansonia. All species of Adansonia except A. digitata are diploid; A. digitata is tetraploid. Section Brevitubae includes A. grandidieri and A. suarexensis. These are species with flower buds that set on short pedicles and that are approximately twice as long as wide. The other species are all classified within the section Longitubae. They also have flowers/fruits set on short pedicels, but the flower buds are five or more times as long as wide.
Species
, there are eight recognized species of Adansonia, with six endemic to Madagascar, one native to mainland Africa and the Arabian Peninsula, and one native to Australia. The mainland African species also occurs on Madagascar, but it is not a native of that island. Baobabs were introduced in ancient times to south Asia and during the colonial era to the Caribbean. They are also present in the island nation of Cape Verde. A ninth species was described in 2012 but is no longer recognized as a distinct species. The African and Australian baobabs are similar in appearance, and the oldest splits within Adansonia are likely no older than 15 million years; thus, the Australian species represents a long-distance trans-oceanic dispersal event from Africa. The lineage leading to Adansonia was found to have diverged from its closest relatives in Bombacoideae like Ceiba /Chorisia at the end of the Eocene, during a time of abrupt global climate cooling and drying, while a divergence of this Adansonia+''Ceiba /Chorisia clade from Pachira was found to be more ancient, dating to the middle Eocene.| Image | Species | Common names | Native range |
| Adansonia digitata L. | African baobab, dead-rat-tree, monkey-bread-tree, montane African baobab, Gongolaze | western, northeastern, central and southern Africa, SW Asia | |
| Adansonia grandidieri Baill. | Grandidier's baobab, giant baobab | west central Madagascar | |
| Adansonia gregorii F.Muell. | boab, Australian baobab, bottletree, cream-of-tartar-tree, gouty-stem | Australia | |
| Adansonia madagascariensis Baill. | Madagascar baobab | northwest and north Madagascar | |
| Adansonia perrieri Capuron | Perrier's baobab | northern Madagascar | |
| Adansonia rubrostipa Jum. & H.Perrier | fony baobab | central-to-south part of western Madagascar | |
| Adansonia suarezensis H.Perrier | Suarez baobab | northern Madagascar | |
| Adansonia za'' Baill. | za baobab | west and southwest Madagascar |