Synoptic Overview of Exotic Acacia, Senegalia and Vachellia (Caesalpinioideae, Mimosoid Clade, Fabaceae) in Egypt

For the first time, an updated checklist of Acacia, Senegalia and Vachellia species in Egypt is provided, focusing on the exotic species. Taking into consideration the retypification of genus Acacia ratified at the Melbourne International Botanical Congress (IBC, 2011), a process of reclassification has taken place worldwide in recent years. The review of Acacia and its segregates in Egypt became necessary in light of the available information cited in classical works during the last century. In Egypt, various taxa formerly placed in Acacia s.l., have been transferred to Acacia s.s., Acaciella, Senegalia, Parasenegalia and Vachellia. The present study is a contribution towards clarifying the nomenclatural status of all recorded species of Acacia and its segregate genera. This study recorded 144 taxa (125 species and 19 infraspecific taxa). Only 14 taxa (four species and 10 infraspecific taxa) are indigenous to Egypt (included now under Senegalia and Vachellia). The other 130 taxa had been introduced to Egypt during the last century. Out of the 130 taxa, 79 taxa have been recorded in literature. The focus of this study is the remaining 51 exotic taxa that have been traced as living species in Egyptian gardens or as herbarium specimens in Egyptian herbaria. The studied exotic taxa are accommodated under Acacia s.s. (24 taxa), Senegalia (14 taxa) and Vachellia (13 taxa). Identification keys for the studied genera, generic groups and species have been provided using different taxonomic criteria. For each taxon, the validated name with the first citation followed by relevant Egyptian citations, typification, synonyms, distinctive features, origin, ecology (when available), utilisation and selected specimens are provided. The study revealed the presence of 22 newly recorded taxa in Egypt. Additionally, a list of excluded, unvalidated and unresolved names is given.

Acacia Mill. s.l. in its traditional circumscription is the second largest genus in Fabaceae (approximately 1503 species according to WorldWideWattle database [4]), widespread in tropical, subtropical and warm temperate areas of the world [1,[5][6][7]. The majority of species are centered in Australia, many in America and Africa, and fewer in Asia [6,8,9].
This speciose genus has received considerable critical attention due to its biocultural, high species number, symbolic and economic significance in Africa, America and Australia [6]. Many of its species are dominant components of drier vegetation, such as the thorn scrubs of southern Africa and the mulga woodlands in Australia dominated by the mulga tree Acacia aneura.
On the other hand, the number of non-indigenous or exotic species which have been introduced to the country due to intentional or unintentional human involvement [37] is about 2000 species without succulents. The Leguminosae includes about 300 species [38][39][40].
By comparing the number of exotic or non indigenous species to native or indigenous species, which are almost similar. It is an equally important goal to carry out careful taxonomic revisions on the exotic species, updating their names and making an effort to define the unknown species, as many of them have been acclimatized and spread in the Egyptian gardens and streets, in addition to being used in many fields.
In earlier floristic studies, little attention was paid to exotic Acacia s.l. species in Egypt. During the reigns of Mohamed Ali, Ibrahim Pasha and Khedive Ismail (19th century), numerous Acacia s.l. species had been introduced and acclimatized over time in Egyptian gardens. More than 100 Acacia species were listed in a considerable amount of literature that dealt with the cultivated plants in Egypt (Appendix B). [38,[41][42][43][44] are among the most important references. Despite this large number of recorded exotic Acacia s.l. species in Egypt, they lack satisfactory data concerning the precise description and real representation in the Egyptian herbaria.
The main goals of this study are to: provide a synopsis of the Acacia, Senegalia and Vachellia species that are introduced to the country; update the status of all recorded Acacia s.l. species in Egypt with the new generic classification and nomenclature; highlight the most important species that have been adapted to the Egyptian environment and are still growing; and provide full annotated appendices for the recorded indigenous and exotic Acacia s.l. species in Egypt (even species cited only in literature).

Results and Discussion
In this study, 144 Acacia taxa (125 species and 19 infraspecific taxa) have been recorded (Appendix A).
Current evidence shows Acacia s.l. to be represented in Egypt by 14 indigenous taxa (four species, 10 infraspecific taxa; see Appendix A) placed in Senegalia, namely: S. asak Acacia nilotica (now Vachellia nilotica) has been known to the ancient Egyptians since pre-dynastic times, and the tree is well documented in inscriptions and wall paintings in tombs. Its wood was used for the manufacture of sarcophagi and agricultural tools, the pods were an essential source of tannins used for tanning and dyeing leather, and its thorns were used as needles.
On the other hand, 130 Acacia s.l. taxa were introduced to Egypt from different regions of the world during the last century to adorn our gardens and streets. The first effort was made in Egypt under the rule of Mohamed Ali Pasha, and it reached its zenith under the reign of Khedive Ismail. Some species have commenced to proliferate rapidly along North Sinai's eastern and western coasts (e.g., A. saligna).   In terms of its economic significance in Egypt, the acacia tree was one of the most useful timber trees, with its sturdy, reddish-colored wood. It was used for making agricultural equipment, ships, furniture, sugar cane presses, ancient rice mills, car wheels as well as in buildings. It was also utilized as a source of fuel. The gum of A. Senegal exudate from the tree is used in medicine, while the flowers of A. farnesina were used for making perfumes and fragrant cosmetics. The higher nutritive value and production biomass of A. saligna make the genus an attractive animal fodder. A. mangium, considered to be an agroforestry tree, was introduced for its wood value and can provide some ecological services such as absorbing pollutants, sequestering carbon, providing shade, and creating oxygen.
All the exotic taxa recorded in Egypt are listed in Appendix B. Out of the 130 taxa, 79 taxa have been recorded in literature. The focus of this study is the remaining 51 exotic taxa (39 species, five subspecies, seven varieties).
The studied taxa are traced as living trees or shrubs growing in Egyptian gardens (36 taxa), preserved herbarium specimens (17 taxa) or newly recorded (23 taxa). The studied taxa are accommodated in the three new genera: Acacia s.s. (24 taxa), Senegalia (14 taxa), and Vachellia (13 taxa). Each of the studied genera is classified into a number of informal groups to facilitate the differentiation. An additional list of excluded, unvalidated and unresolved names is given. The identification keys of the studied genera and generic groups are based mainly on the leaf structure followed by the presence of spiny stipules or prickles and type of inflorescence.
Key to identify the studied genera 1A. Leaves  Woody trees or shrubs, rarely lianas; branches rarely with prickles. Leaves bipinnate or modified to polymorphic phyllodes, rarely reduced to scales or absent; foliar glands normally present; stipules present, but commonly caducous and scarious, rarely spinose. Inflorescences simple, racemose or paniculate; flowers white to golden, rarely pink, 2 or more aggregated into globular heads or cylindrical spikes, (3-) 4 or 5 (-6)-merous; sepals free to united, rarely absent; stamens numerous, free, rarely united basally into a short tube; ovary sessile or rarely stipitate. Legumes dehiscent or rarely indehiscent.
According to WorldWide Wattle website [4], Acacia s.s. has 1080 species, mainly restricted to Australia (1075 in Australia,12 in Asia), making it the largest genus of vascular plants on that continent [9]. In Egypt, no native Acacia s.s. species are recorded. In the present study, 24 exotic taxa (23 species, one subspecies) are recognized. They are accommodated in six informal groups (A-F).
Key to identify the studied Acacia groups  [56], species of the Mulga group (A. aneura and its close relatives) are enormously variable. Their identification in the field and from herbarium material is often challenging. Therefore, there is ambiguity concerning the application of the name A. aneura because of uncertainties regarding the taxon's identity [57]. The taxonomic revision of Mulga group carried out by [56] in Western Australian defined 12 species based on morphological characters (branchlet resin; pod margins; phyllode shape, size, curvature; pod width).
Native to Australia (New South Wales, Northern Territory, Queensland, South Australia, Western Australia). It was introduced to Egypt in the early 1920s from Australia [44]. It was growing very well in the Egyptian environment and listed among the seeds available for exchange [58].
Utilisation: Seeds have been used as a food source by Aboriginal people. The wood has been used by indigenous people of the Pilbara for weapons, walking sticks and tapping sticks [59]. In Egypt, it is cultivated as an ornamental tree in gardens. It is considered among the best-known Australian acacias introduced to North Africa many years ago [60].  Ecology and utilisation: Its rapid growth rate, ability to fix nitrogen, tolerance to variable soil types make it a beneficial species for rehabilitation of degraded lands. It has been widely planted for fuelwood production, erosion control, ornament or shade mainly in Asia, Africa and South America. The wood is suitable for construction works (e.g., framing, flooring), woodturning and carving. The bark tannins are used for commer-cial exploitation [50]. In Egypt, it is cultivated as an ornamental tree with its showy flowers in some gardens.  [45].
Utilisation: It has potential for use as a stock food during drought, and is useful for site rehabilitation after sand mining [50]. In Egypt, it is cultivated as an ornamental tree in some gardens. Native to Australia (Queensland, South Australia, Tasmania, Victoria). It was introduced to Egypt in the 19th century, under the name A. sophorae [41].
Ecology and utilisation: It is known for its nitrogen-fixing ability, so used in coastal dune restoration and rehabilitation. It is recorded as invasive according to the Global Invasive Species Database [52]. In Victoria and South Australia, it has become a serious environmental weed of near-coastal heaths and woodlands. It has also become a very serious problem within its geographic native range in the last two decades [61,62]. In Egypt, it is cultivated as an ornamental tree in historical gardens. Distinctive features: Tree 7-30 m high. Bark corrugated or coarsely cracked, grey to dark brown. Phyllodes narrowly elliptic to elliptic, 11-27 cm × 23-95 mm, coriaceous or papery, glabrous, with 3 or 4 (-5) main prominent nerves (confluent at base of phyllodes near lower margin). Spikes 5-12 cm long, loosely arranged, white to cream-coloured. Flowers 5-merous. Pods linear, openly coiled and twisted or tightly spirally coiled, 3-5.5 mm wide, coriaceous to subwoody, glabrous.
Native to Australia (Queensland); Southeast Asia (Indonesia, Papua New Guinea). It was introduced to Egypt in the 2000s [63].

Ecology and utilisation:
It is recorded as invasive according to the Global Invasive Species Database [52]. It is a significant reforestation species in tropical Asia. The leaves can serve as forage for livestock. Heartwood is suitable for furniture, cabinet making, agricultural tools, boxes and crates, while mature wood is used as a good fuel [50]. In Egypt, the tree has an excellent potential to restore soil fertility, so it is currently cultivated in agroforestry systems in Serapium forest (Ismailia) for its wood and in some gardens as an ornamental tree.  [41,43].
Ecology and utilisation: It is regarded as an environmental weed in Tasmania and Western Australia. It has become naturalised outside its native range in California, and is classified as a noxious weed in this state. It has been planted as a hedge [64,65]. In Egypt, it is cultivated as an ornamental tree in historical gardens.  [45].
Utilisation: It is a useful stock food supplement during droughts and for soil stabilisation in dry countries. It also contains compounds called avicins, which may have medicinal uses [66]. It has been successfully used in Israel and Libya under 150-200 mm of rainfall. It also has a feed value comparable to A. saligna [60]. In Egypt, it is cultivated as an ornamental tree in some private gardens. Distinctive features: Shrub up to 4 m high. Branchlets completely glabrous. Phyllodes crowded, inequilateral, often ±triangular, with abaxial margin ±straight and adaxial margin markedly rounded or angled, 10-30 × 5-15 mm, coriaceous, grey-green to glaucous, glabrous; midrib central or excentric. Racemes prolific in upper axils; axes 1-5 cm long, glabrous; heads globular to shortly cylindrical, 13-40-flowered, bright golden. Pods narrowly oblong, up to 9 cm long, (4-) 5-8 mm wide, chartaceous, often pruinose, glabrous.
Native to Australia (New South Wales, Queensland). It was introduced to Egypt in the 19th century, under the name A. armata [41,43] and listed among the seeds available for exchange [58].
Native to Australia (Australian Capital Territory, New South Wales, South Australia, Victoria). It was introduced to Egypt in the early 1920s from Australia [44].
Ecology and utilisation: It is a crucial environmental weed in Western Australia, Tasmania and parts of New South Wales outside its native range. In South Africa, it competes with and replaces indigenous species. It is widely planted as an ornamental plant. The bark is one of the richest sources of tannin globally, although it is now rarely used commercially [70,71]. In Egypt, it had been cultivated in botanical gardens in the past. Notes: According to [72], Acacia retinodes is a member of the 'Acacia microbotrya group' and is most closely related to A. provincialis and A. uncifolia. He used many morphological characters (habitat, habit, bark, phyllode characters, flower characters and pod width) to differentiate between the three species. In the present work, the phyllode characters and pod width are considered the most significant characters.
Native to Australia (South Australia). It was introduced to Egypt in the 19th century [41,43].
Ecology and utilisation: It is recorded as invasive according to the Global Invasive Species Database [52]. It is regarded as an environmental weed in Victoria and had become locally naturalised outside its native range [73,74] noted that A. retinodes has good prospects for future cultivation and development, for wood, tannin, fodder, seed and gum products. In Egypt, it is cultivated as an ornamental tree in historical and some private gardens. Distinctive features: Shrub or tree 3-13 m high, often clonal due to suckering habit. Branchlets completely glabrous. Phyllodes pendulous, variable, linear to narrowly oblanceolate or narrowly elliptic, 7-20 cm × 4-30 mm, green to grey-green, glabrous, 1-veined, penninerved. Inflorescences 2-8-headed racemes; raceme axes 1-5 cm long, glabrous, rarely appressed-puberulous; heads globular, 15-25-flowered, cream to pale yellow. Pods narrowly oblong, up to 12 cm long, 7-13 mm wide, woody, thick, longitudinally striate when dry, grey-green, glabrous.
Native to Australia (New South Wales, Northern Territory, Queensland, South Australia, Victoria). It was introduced to Egypt in the early 1920s from Australia [44].
Utilisation: The pendulous habit and attractive foliage make it a desirable species for amenity planting. In Australia, sheep eat the leaves and pods. It has been planted as a fodder species in the arid zone of Libya and showed promise in semi-arid areas of Iran and Kuwait. The heartwood has been used for quality furniture. It is also used for shade, shelter, and ornamental purposes in North Africa and the Middle East. It suckers freely and can stabilize sandy areas and control erosion along stream banks [50]. In Egypt, it had been cultivated in botanical gardens in the past. Distinctive features: Shrub or tree (1-) 3-10 m high, often root-suckering. Bark grey, texture variable. Branchlets completely glabrous. Phyllodes patent to pendulous, linear to lanceolate, straight to falcate, 10-25 cm × 5-35 mm, green to glaucous, glabrous, with prominent midrib, finely penninerved. Inflorescences 2-10-headed racemes; raceme axes 2-60 mm long, glabrous; heads globular, 25-55-flowered, golden to lemon yellow. Pods linear, flat, shallowly constricted between seeds, 8-12 cm × 4-6 mm, coriaceous, glabrous.
Notes: It is a highly polymorphic species comprising four informal variants that have been assigned subspecies rank in some genetic studies [74][75][76]. It is currently under taxonomic review [77].
Native to Australia (Western Australia). It was introduced to Egypt in the 19th century [41,43], and listed among the seeds available for exchange [58].
Ecology and utilisation: It is a crucial environmental weed in the south-eastern parts of Australia and South Africa. It is recorded as invasive according to the Global Invasive Species Database [52]. It is a very adaptable species that can tolerate relatively dry, low nutrient soils and thrive in better conditions. It is planted widely for a range of purposes (e.g., windbreaks, sand dune fixation, fuelwoods, fodder production) in West Asia, South America, North and South Africa and parts of the Mediterranean [77,78]. In Egypt, it was cultivated as an ornamental tree in historical and many public gardens and streets. Recently, 1 million seedlings have been planted along the Mediterranean coast of Egypt for a range of rehabilitation [79]. It escaped from cultivation, spread invasively, and strongly affected biodiversity [80]. Currently, it is utilised in forest plantations in Gebel Elba natural forests, for sand dune fixation, as a bio-fertiliser and windbreaks. Tannin material extracted from the bark is used in tanning. Leaves, twigs, and fruits are used as fodder for livestock. Flowers are utilised in bee-keeping for honey production. Wood is used as firewood and in charcoal making [81].  Distinctive features: Bushy shrub up to 4 m high. Branchlets hairy to some degree. Phyllodes inequilaterally ovate-elliptic, 10-20 × 4-10 mm, grey-green to ± glaucous, 1-veined per face. Inflorescences racemose; raceme axes 1.5-6 cm long, hirtellous; heads globular, 12-18-flowered, bright light golden. Pods narrowly oblong, rounded over seeds, up to 11 cm long, 10-14 mm wide, coriaceous, dark brown, glabrous, pruinose when young.
Native to Australia (New South Wales). It was introduced to Egypt in the 19th century [42].
Native to Australia (New South Wales, South Australia). It was introduced to Egypt in the 19th century [43].
Native to Australia (South and Western Australia). It was introduced to Egypt in the 20th century [38].
Ecology and utilization: A significant environmental weed outside its native range within Australia and a potential threat in Victoria because of its invasiveness and significant environmental impacts elsewhere. It is also troublesome in South Africa, where it suppresses the indigenous vegetation and reduce species diversity. It is a drought-tolerant species and more tolerant to sea spray. Therefore, it is grown mainly to stabilise coastal sand dunes in North Africa (Tunisia, Libya, and Egypt). It also produces a dense, high-quality fuelwood [84,85]. In Egypt, it was cultivated as an ornamental tree in gardens. Distinctive features: Tree up to 25 m high, root-suckering. Bark furrowed, almost black. Phyllodes falcate, 10-20 cm × 7-20 mm, coriaceous, sericeous, with numerous closely parallel veins of which 3-7 are more prominent than the rest. Inflorescences 2-8-headed racemes; raceme axes 2-10 mm long, appressed-puberulous; heads globular, 5-8 mm diam., 15-35-flowered, golden. Pods subterete, slightly constricted between seeds, straight to curved, up to 20 cm long, 5-10 mm wide, crustaceous, longitudinally veined, glabrous.
Native to Australia (New South Wales, Queensland). It was introduced to Egypt in the 20th century [45].
Ecology and utilization: It can form dense stands of root suckers when damaged, becoming a weed in some circumstances and is generally considered an undesirable agricultural and rangeland species. The timber has been used for firewood and fence posts [86]. In Egypt, it had been cultivated in botanical gardens in the past. Distinctive features: Tree 6-30 (-45) m high; sometimes a shrub 1.5-3 m high; may spread by root suckers. Bark fissured and scaly. Phyllodes narrowly elliptic, lanceolate or oblanceolate, inequilateral, straight to ± falcate, 4-16 cm × 6-30 mm, obtuse to acute, ± coriaceous, dark green, glabrous, with 3-5 main veins and prominently reticulate in between. Inflorescences 3-5-headed racemes; raceme axes 6-40 mm long; heads globular, 6 mm diam., 30-56-flowered, creamy pale yellow to white. Pods strongly curved to openly coiled and often twisted, up to 15 cm long, 3.5-8 mm wide, coriaceous to subwoody, glabrous.
Native to Australia (Australian Capital Territory, New South Wales, Queensland, South Australia, Tasmania, Victoria). It was introduced to Egypt in the 19th century [41,43].
Ecology and utilisation: It is mainly cultivated in Australia as an ornamental species in wetter areas. It has also been cultivated in forestry plantings in Hawaii, New Zealand, South Africa, Zimbabwe and Sri Lanka [87,88]. It is recorded as invasive according to the Global Invasive Species Database [52]. It has the potential to become naturalised in places where it is cultivated. In South Africa, the species has become an environmental weed difficult to control because of its fast growth rate, vigorous regrowth from root suckers and regeneration from seeds. In Egypt, it had been cultivated in historical and some public gardens in Cairo and Alexandria in the past [51]. According to [38], it is a valuable tree produces excellent timber used for furniture, railway carriages and building purposes. Erect or climbing shrubs, lianas, or trees, with terete to angled branches. Stems striate, glabrous or with varied indumentum, armed with prickles. Stipules persistent or caducous. Leaves bipinnate, with sessile or stipitate extrafloral nectaries on the petiole or occasionally on the leaf and pinnae rachises. Inflorescences usually panicles, with flower-bearing axis capitate or spicate; floral bracts generally deciduous. Flowers whitish to yellowish, usually pentamerous, rarely tetramerous or hexamerous; calyx gamosepalous, glabrous or pilose; corolla gamopetalous, glabrous or pilose; nectarial disc present; stamens numerous, filaments free or fused at the base, anthers rimose, tipped by a caducous gland; ovary superior, stipitate, glabrous or pilose, stigma punctiform. Fruit legume, rarely follicle, flat, straight or falcate, chartaceous to coriaceous.
Senegalia occurs widely in the tropics and subtropics of both the Old and New Worlds [28], with its center of diversity in the neotropical region. According to WorldWide Wattle website [4], it has 217 species (two species in Australia, 56 in Asia, 62 in Africa, 97 in the Americas). In Egypt, only three native Senegalia taxa (two species, one subspecies) are present. In this study, 14 exotic taxa (eight species, three subspecies, three varieties) are recognized. They are accommodated in three informal groups (A-C).
Native to Africa (Botswana, Mozambique, South Africa, Swaziland, Zimbabwe). It was introduced to Egypt in the 20th century [45].
Ecology and utilisation: It is found on sandy soils in hot and dry deciduous woodland. It secretes nectar, and its flowers attract many visitors. People, monkeys and bushbabies eat its gum. Bark and roots are used in traditional medicine to treat eye and back complaints. The wood is used to make furniture, tool handles and long-lasting fence posts. The heartwood makes a good quality fuel with coals that burn for a long time. Dry pods have a high nutritional value and are eaten by cattle [89]. In Egypt, it is cultivated as an ornamental tree in some private gardens. Native to Indian Subcontinent (Bangladesh, Bhutan, India, Nepal, Pakistan); Southeast Asia (Myanmar). It was introduced to Egypt in the 19th century [41].
Ecology and utilisation: It is recorded as invasive according to the Global Invasive Species Database [52]. Extracts are used medicinally for a range of ailments. Tannins are used as a dye and for leather tanning. Wood for building construction and other purposes [26]. In Egypt, it had been cultivated in historical and some public gardens in Cairo and Alexandria [51].   Prickles in pairs just below the nodes, straight or recurved, up to 1 cm long. Leaves bipinnate; rhachis subglabrous to ± puberulous or pubescent, glandular between the top 1-4 pairs of pinnae; pinnae (4-) 9-14 pairs; leaflets 12-45 pairs, oblong to linear-oblong, obtuse to subacute. Inflorescence spicate; spikes (4-) 5-11 cm long; flowers sessile; calyx purple or reddish-purple, 0.75-1.25 mm long, glabrous. Pods purplish-brown, dehiscent, 11.5-28 × 2.7-3.5 cm, straight, glabrous.
Native to Africa (Botswana, Malawi, Mozambique, South Africa, Tanzania, Zambia, Zimbabwe). It was introduced to Egypt in the 20th century [45].
Ecology and utilisation: This tree can survive harsh conditions and makes a stunning tree along roads where there is enough space. Many insects such as bees and wasps visit the flowers. In the wild, the plant is grazed by different animals and used for shade during the hot summer [90]. In Egypt, it is cultivated as an ornamental tree in some private gardens. Distinctive features: Tree 3-20 m high. Bark rough, grey or brown. Stipules not spinescent. Prickles in pairs just below the nodes, pale then dark-brown or grey, up to 7 mm long, hooked downwards. Pinnae 3-10 pairs; rhachis with a gland between the topmost pair of pinnae only; leaflets in 3-5 pairs, obovate, obovate-oblong or oblanceolateoblong. Inflorescence spicate; spikes (2-) 3-12 cm long; flowers sessile, white or slightly yellowish; calyx glabrous. Pods dehiscent, (5-) 8-18 × 2-3.5 cm, glabrous, oblong or irregularly constricted, venose, red-to purplish-brown, acuminate or apiculate at apex.
Native to Africa (Angola, Democratic Republic of Congo, Kenya, Malawi, Mozambique, Somalia, Tanzania, Zambia, Zimbabwe). It was introduced to Egypt in the 20th century [45].
Native to Indian Subcontinent (India, Pakistan); Southeast Asia (Myanmar); West Asia (Afghanistan). It was introduced to Egypt in the early 1920s from East Indies [44].
Ecology and utilisation: It is a slow-growing tree, succeeds in dry and shallow soils. The established plants are very drought tolerant. Its gum is restorative, stimulant, tonic, and can treat muscular conditions, back pain, and stomach problems. The plant is used to treat dysentery, leprosy, oral toothache, trachoma, venereal diseases, and wounds. The tree is used as a pioneer for reforestation projects in arid and semi-arid locations. It is also grown to provide shelter from the wind [92]. In Egypt, it has occasionally been cultivated in botanical gardens and still growing in some gardens (e.g., Manial Palace).  [45].
Native to Africa (Democratic Republic of Congo, Ethiopia, Kenya, Sudan, Uganda). It was introduced to Egypt in the 20th century [45].
Ecology and utilisation: It is growing on woodland, wooded grassland and savanna on quartzite soil [95]. In Egypt, it had been cultivated in botanical gardens in the past.    [45].
Ecology and utilisation: It is a very fast-growing tree, short-lived (30-40 years), sensitive to cold and frost, growing on wooded grassland, deciduous woodland and bushland, and open woodland with good grass cover [96]. The gum is used in confectionery products and as an adhesive, while the bark is used in tanning. The roots and bark are used for medicinal and magical purposes [97]. In Egypt, it is cultivated as an ornamental tree in some gardens. It grew well in botanical gardens in the past, under the name Acacia suma, but unfortunately, no specimens are left to identify the subspecies [38,44].  (1960).
Ecology and utilisation: It grows in drier areas, woodlands, wooded grassland, sometimes in Lowveld in riverine areas forming thickets [100]. It is resistant to decay due to gum deposits. The wood can be split into paper-like strips without cracking, and these strips are commonly used as weaving material for baskets. The roots are also used in basketry and in making long-stem tobacco pipes. It is used in traditional medicine to treat constipation and abdominal pains [101]. In Egypt, it is cultivated as an ornamental tree in some private gardens.  (Mexico); North America (Arizona, California, Nevada, New Mexico, Texas, Utah). It was introduced to Egypt in the 20th century [38].
Utilisation: It is one of the most common and iconic desert scrub plants where found. Its flowers are one of the most important nectar sources for honey bees [102]. In Egypt, it had been cultivated in botanical gardens in the past. Notes: According to [26], the disagreeable odour that is emitting from fresh leaves or branchlets when crushed, and leaflets that often obviously curved forward and/or folded lengthwise when dry are the most distinctive features for this subspecies in this study.
Native to East Asia (China); Southeast Asia (Myanmar). It was introduced to Egypt in the 20th century [45].
Utilisation: The soft new shoots are commonly used in Asian cooking and as a hedgerow shrub in Thailand [26]. According to [103], the roots of this subspecies in Laos are used in local medicine to combat anemia. In Egypt, it is cultivated as an ornamental tree in some private gardens. Endemic to Madagascar. It was introduced to Egypt in the 2000s [45]. Utilisation: In Madagascar, powder from grated stems is rubbed around forehead to cure headache. Leaf tisanes are taken to cure diarrhoea [104]. In Egypt, it is cultivated as an ornamental tree in some private gardens. Trees or shrubs, sometimes climbing, always armed with spiny stipules, situated near the leaf bases. Leaves alternate, bipinnate, with opposite pinnae. Inflorescences racemose; flowers bisexual, rarely unisexual, yellow or creamy white, in spherical heads, or rarely elongated spikes; calyx and corolla 4 to 5-lobed; stamens numerous. Glands usually present on the rachis and the upper side of the petiole. Pods straight, curved or curled, dehiscent or indehiscent.
Vachellia is a wide-ranging genus, has a pantropical distribution. According to World-Wide Wattle website [4], it has 164 species (nine species in Australia, 33 in Asia, 72 in Africa, 61 in the Americas). In Egypt, only 11 native taxa (two species, six subspecies, three varieties) are recorded. In this study, 13 exotic taxa (eight species, one subspecies, four varieties) are recognized. They are accommodated in three informal groups (A-C).
Native to South America (Argentina, Bolivia, Chile, Paraguay, and Uruguay). It was introduced to Egypt in the 19th century [41,43].
Ecology and utilisation: A fast-growing plant succeeds in warm temperate to subtropical and tropical areas, grows well in slightly acidic to acidic soil with a significant content of organic matter. Drought tolerant but cannot withstand temperatures below freezing. It can be used as a hedge and in planting programs for restoring native woodland.
The seeds can be toasted and used as a coffee replacement. The cooked bark can heal sores and wounds [105]. In Egypt, it had been cultivated in historical gardens in the past.
Ecology and utilisation: It is an attractive shrub with fragrant, showy orange-yellow flowers that attract nectar-seeking insects, butterflies and bees. It is well adapted to the arid conditions of desert life, and prefers dry slopes, washes, flat desert areas and mesas. Livestock will consume the fruit pods but do not appear to relish the foliage [106]. In Egypt, it had been cultivated in gardens (e.g., Orman garden) as an ornamental shrub in the past. Native to Africa (Angola, Botswana, Namibia, South Africa, Zambia, Zimbabwe). It was introduced to Egypt in the 20th century from South Africa under the name "camel thorn" [38].
Ecology and utilisation: It is a very highly invasive species in Australia that can displace preferred vegetation. It is a long-lived plant that grows in sandy areas and can tolerate hot summer temperatures and severe frosts. The pods are useful fodder for cattle and wild animals in Africa. Dry powdered pods can be used to treat ear infections, gum to treat gonorrhea, burned bark to treat headaches and root to treat toothache [107]. In Egypt, the wood was used as fuel [38]. It is cultivated as an ornamental tree in some gardens [45].  Ross, 1975).
Native to Central America (Belize, Guatemala, Honduras, Mexico; North America (California, Florida, Louisiana, Texas). It was introduced to Egypt a long time ago, probably from tropical America [108], then cultivated and become naturalized.
Ecology and utilisation: It is recorded as invasive according to the Global Invasive Species Database [52]. The foliage and green pods are palatable to cattle and sheep. It is favoured for its fragrant flowers and cultivated in Mediterranean countries for its essential oil to manufacture perfumes. Although the growth rate is slow, it is valuable ornamentally and used as hedges around citrus groves, especially in Egypt and Israel [109,110]. Its flowers produce a perfume called "cassie" extensively used in European perfumery. In Egypt, the bark and pods are used for dying and tanning leather [111], and the wood is mainly used for fuel [38].
Native to Africa (Kenya, Malawi, Mozambique, Somalia, South Africa, Swaziland, Tanzania, Zimbabwe). It was introduced to Egypt in the 20th century from South Africa, under the name "Fever tree" [38].
Ecology and utilisation: This tree is popular among birds for nest building as the thorns add extra protection against predators such as snakes. Young branches and leaves are eaten by elephants, while giraffes and monkeys eat the leaves and pods. Also, baboons eat the gum and green seeds. Medicinally the bark is used for treating fevers and eye complaints [117]. In Egypt, it is cultivated in gardens as an ornamental tree.

Materials and Methods
The scope of this study includes all species of the former broadly circumscribed genus Acacia that occur in Egypt.
The morphological investigation was based on the herbarium specimens kept in the following Egyptian herbaria: Cairo University (CAI), Agricultural Research Center (CAIM), Mazhar Botanical Garden (MAZHAR) and Orman Botanical gardens. Additionally, fresh specimens of cultivated Acacia s.l. species collected during many field trips conducted between 2018 and 2019 in most public Egyptian gardens and some private botanical gardens were examined. Reliable characters for species delimitation were observed (e.g., leaf structure, types of stipule, inflorescence, flower colour, pod characters). Species were identified with the help of digital photographs of the authentic specimens kept at virtual herbaria available on-line (WorldWideWattle database [4]; (K) Kew Royal Botanic Garden [123]; Tropicos [124]; the JSTOR Global Plants database [125]; (POWO) Plants of the World Online [126]). The identifications of Australian Acacia s.s. species were checked by consulting the free on-line WATTLE identification key [127], and the Flora of Australia Online (FOAO) treatments [128].
Currently accepted name, type, accepted synonyms, distinctive features (based on our observations on the Egyptian plants and data collected from literature), origin, ecology (when available), utilisation and selected specimens were given for each studied taxon. Accepted names and synonyms were validated from the WorldWideWattle database [4]. Photographs of some living species found in this study were provided.
Abbreviations of taxon author and literature names follow those given in IPNI (International Plant Name Index) [135].
Type specimens were viewed as digital images obtained from JSTOR Global Plants database [125] or quoted from literature. Type terminology follows the International Code of Nomenclature [136]. Types that were seen by the authors are followed by (!).
The most commonly used synonyms in Egypt are included. The following symbols have been used for synonyms: = denotes a heterotypic synonym; ≡ denotes a homotypic synonym.
Selected voucher specimens from Egyptian herbaria were given with institution accession numbers when available. Acronyms of herbaria follow Index Herbariorum [137].
All Acacia s.l. names recorded in Egyptian taxonomic literature for indigenous and exotic species are included in Appendix A, respectively, with the currently accepted names. The exotic species are studied in some detail below. The species are arranged alphabetically under their respective group and numbered sequentially.