Acacia nilotica

Name and classification

Acacia nilotica (L.) Willd. ex Delile
(=Mimosa nilotica L.; Acacia arabica (Lam.) Willd.)
Family Fabaceae, Subfamily Mimosoideae

Common names

prickly acacia

Disseminule

fruit; one-seeded fruit segment

Description

Fruit a coriaceous, indehiscent, sweet-smelling legume. Pods variable (see subspecies descriptions below), oblong to linear, compressed but thick, 4-25 cm long, 10-22 wide, dark brown to gray, straight or curved, margins straight to deeply constricted (moniliform) between seeds, surface glabrous to velvety (causing fruit to appear whitish in the later case), not reticulately striate, stipe to 15 mm long, apex acuminate beaked. Seed chambers visible, number 6-17. Seeds uniseriate and oriented transversely.

Seeds oblong to obovate to circular, flattened, umbo absent, (5.0)6.5-9.0(10.4) mm long, (4.5)5.0-8.0 mm wide, 2.5-4.2(4.8) mm thick, oblong to elliptic in cross section, umbo absent, yellowish-brown to dark brown to brownish black, surface texture undulate to reticulate to granulate (pitted), somewhat shiny. Pleurogram an oblong to circular O-shape, sometimes open at base, large, measuring (3.0)5.0-7.0(7.5) mm long, 3-6(7) mm wide. Lens elliptic or oblanceolate to obtriangular, flat to mildly raised, yellow. Hilum subterminal to terminal, often with funiculus remnant still attached.

Identification considerations

The distinctive seeds and pods, especially when the margins are deeply constricted in the latter, distinguish this species from others in the genus. The subspecies (see below) most often found as exotics are those with moniliform fruits.

Infraspecific taxonomy

Use this table to distinguish fruits of the nine subspecies of Acacia nilotica.

Subspecies Distribution (native, naturalized & cultivated) Pod Pubescence Pod Shape & Margins
adstringens (Schumach. & Thonn.) Roberty northern tropical Africa east to India densely tomentose not moniliform, 13–21 mm wide, margins distinctly and often irregularly crenate
cupressiformis (J. L. Stewart) Ali & Faruqi Pakistan, India gray-white tomentellous moniliform, narrowly and regularly constricted between the seeds
hemispherica Ali & Faruqi Pakistan subglabrous with very short inconspicuous puberulence not moniliform, narrow (11–13 mm wide), margins slightly crenate to straight
indica (Benth.) Brenan Angola, Tanzania north to southern Arabian Peninsula and east to Myanmar, Vietnam, Australia densely white tomentellous moniliform, narrowly and regularly constricted between the seeds
kraussiana (Benth.) Brenan southern Africa to southern Arabian Peninsula pubescent becoming glabrescent and, later, shiny black on raised areas over the seeds not moniliform, oblong, 10–19 mm wide, margins +/– shallowly crenate
leiocarpa Brenan coastal eastern Africa from Ethiopia south to Mozambique glabrous or almost so, rarely slightly puberulous not moniliform, oblong, narrow (10–13 mm wide), margins straight or slightly crenate
nilotica northern tropical Africa from Senegal to Egypt, Tanzania, Iraq, southern Arabian Peninsula glabrous or almost so moniliform, narrowly and regularly constricted between the seeds
subalata (Vatke) Brenan Tanzania north to Sudan and then east to India and Sri Lanka densely subtomentose not moniliform, 15–22 mm wide, margins straight or slightly crenate
tomentosa (Benth.) Brenan northern tropical Africa gray-white minutely tomentose moniliform, narrowly and regularly constricted between the seeds

Distribution

native to tropical and subtropical Africa and Asia east to India

naturalized in China, Australia, and various oceanic islands; introduced to the United States (Arizona, California, Florida, Puerto Rico, Virgin Islands) various times over the years

Habitat

Depending on subspecies, these plants occur in grasslands and savannas, dry forests, dry stream beds, or along river drainages.

General information

Acacia nilotica is a spiny shrub. Reproduction is entirely sexual. Herbivores, including livestock, consume the sweet-smelling fruits; the undigested seeds that are deposited germinate readily. Pods can float, and may be dispersed along stream channels and flood plains in this manner. Seeds may be spread short distances in mud packs adhering to animal hooves. International transport of these economically useful species is mostly intentional; seeds or fruits are carried by passengers or sent through the mail from organizations specializing in medicinal or ornamental plants.

seeds

seeds

lens and funiculus remnant of seed

lens and funiculus remnant of seed

A, seed; B, embryo in situ; C, transection of seed; drawing by Lynda E. Chandler

A, seed; B, embryo in situ; C, transection of seed; drawing by Lynda E. Chandler

legumes of A. nilotica ssp. kraussiana; photo: © C. J. Botha, University of Pretoria, South Africa

legumes of A. nilotica ssp. kraussiana; photo: © C. J. Botha, University of Pretoria, South Africa

seed removed from single-seeded section of pod of A. nilotica ssp. kraussiana; courtesy EcoPort (www.ecoport.org): M. Jooste

seed removed from single-seeded section of pod of A. nilotica ssp. kraussiana; courtesy EcoPort (www.ecoport.org): M. Jooste

herbarium specimens showing moniliform (subspecies nilotica) and non-moniliform (subspecies adstringens) fruits of A. nilotica; photos & specimens: © The Board of Trustees of the Royal Botanic Gardens, Kew

herbarium specimens showing moniliform (subspecies nilotica) and non-moniliform (subspecies adstringens) fruits of A. nilotica; photos & specimens: © The Board of Trustees of the Royal Botanic Gardens, Kew

fruits and seeds of A. nilotica ssp. adstringens from Senegal; photo: © L. Toussaint, www.mauritanie-decouverte.net

fruits and seeds of A. nilotica ssp. adstringens from Senegal; photo: © L. Toussaint, www.mauritanie-decouverte.net

plum-headed parakeet feeding on A. nilotica ssp. cupressiformis; photo: © J. M. Garg http://en.wikipedia.org/wiki/User:Jmgarg1

plum-headed parakeet feeding on A. nilotica ssp. cupressiformis; photo: © J. M. Garg http://en.wikipedia.org/wiki/User:Jmgarg1

immature pods of A. nilotica ssp. leiocarpa which will turn black when mature, Kenya; courtesy EcoPort (www.ecoport.org): C. W. Fagg

immature pods of A. nilotica ssp. leiocarpa which will turn black when mature, Kenya; courtesy EcoPort (www.ecoport.org): C. W. Fagg

Acacia nilotica ssp. subalata from Madagascar; photo: © D. Du Puy, Royal Botanic Gardens, Kew

Acacia nilotica ssp. subalata from Madagascar; photo: © D. Du Puy, Royal Botanic Gardens, Kew