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The following is a brief description of the distribution and habitat, use, and morphology and anatomy of the Prosopis species found in Mexico and Latin America. Plants are arranged alphabetically by species names.

6.1 Prosopis alba Gries.

This species is found scattered in the semi-arid areas of Peru. Legumes are large and fleshy and make excellent livestock food. The wood is valuable for food and construction material.

6.1.1 Morphology and anatomy

Stems and roots. Species can develop into a large tree up to 15 m tall with a short trunk up to one meter in diameter. The top of the tree is rounded with drooping branches. Spines, developed from hardened stipules, are paired, small and scarce, and found only on strong shoots.

Leaves. Leaves are large with one to three pairs of pinnae, 6 to 14 cm long. Pinnae support 25 to 50 pairs or leaflets which are linear, acute to subacute, glabrous, 5 to 17 mm long and 1 to 2 mm wide. Leaflet pairs are separated along the rachis at distances of 1.5 to 6 mm.

Inflorescence. The inflorescence is a stubby spikelike raceme 7 to 11 cm long. The calyx is 1 mm long and slightly downy. Stamens and pistil extend above the corolla, which is 3 to 3.2 mm long.

Legume and seed. The legume is sickle or ring-shaped, with a diameter of about 7 cm; it is 12 to 25 cm long, 11 to 20 mm wide and 4 to 5 mm thick, with 12 to 30 segments that are broader than long.

6.2 Prosopis alba var. panta Gries.

The species is found in southern Bolivia and (presumably) northern Chile. Its uses are similar to those of the typical variety. It differs from P. alba in having almost straight legumes, (some of which may reach 30 cm in length), and in its more obtuse leaflets and redish branches.

6.3 Prosopis articulata S. Watson

This species occurs in a limited area near Guaymas, in the state of Sonora, Mexico, on stony mesas or plains and along the Gulf Coast of the Baja peninsula. The plant is not an important economic species and has limited use as fuel.

6.3.1 Morphology and anatomy

Stems and roots. The plant is a many branched, upright shrub 2 to 5 m tall. The trunk is short with smooth, brown bark, and the branches are greenish. Spines are axillary and paired, 0.3 to 3 cm long.

Leaves. Leaves most often have a single pair of pinnae, sometimes two, Petioles are 0.2 to 2.5 cm long, and the pinnae 0.4 to 6.5 cm long. Leaflets are 2.5 to 12 cm long, 1.2 to 3.5 mm broad, linear-elliptical in shape and very slightly downy or glabrous. They are spaced along the rachis at distances slightly less than their own width.

Inflorescence. Racemes are 4 to 9 cm long and are as long or longer that the leaves. Florets are small, numerous and yellow in color. Petals are free, 3.5 to 4 mm long and hairy at the apex. The ovary is pubescent.

Legume and seed. The legume is dry, linear but very much flattened, yellow or tinged with violet, 10 to 24 mm long and 5 to 8 mm wide. Segments are elliptical, spindle shaped, 1 to 1.8 cm long with constrictions 1 to 5 mm broad. The exocarp is very slightly downy or glabrous. The mesocarp is longitudinally fibrous and the endocarp is thin, flexible, smooth and whitish in color. Seeds are longitudinally elliptic, 7 to 8 mm long and brown. The fruit is not sweet, as in common for the genus, but is bitter from which comes its common name, “Mesquite amargo”.

Remarks. This is a rather distinct species, but in some cases, it has most likely has hybridzed with P. glandulosa and P. velutina.

6.4 Prosopis burkatii Munoz

This species is found in northern Chile in Tarrapaca province, but not widely distributed. Its uses are unknown.

6.4.1 Morphology and anatomy

Stem and roots. Species is a large, densely and intricately branched creeping shrub 0.5 to 1.5 m tall with spiny, ashy-colored stipules, 0.5 to 3 cm long.

Leaves. The leaves have a single pair of pinnae, 2.5 to 3.5 cm long on a petiole 3 to 5 mm long. There are 7 to 12 pairs of elliptical-oblong leaflets on each pinna; they are glabrous, 4 to 7 mm long and 1 to 2 mm broad.

Inflorescence. Inflorescences are stubby cylindrical spikes up to 3 cm long and 1.5 cm in diameter. Flowers are sessile. Calyx is minutely downy and 3 mm long. Corolla is 4 to 5 mm long. Petals are acute, glabrous outside, and hairy at the inner apex.

Remarks. The species may have originated as a hybrid cross section between P. tamarugo and P. strombulifera, both of which grow in the same region.

6.5 Prosopis chilensis (Molina) Stuntz Emend, Burkart

This species is found in south-central Peru, north-central Chile. It grows in lower elevations and along washes, commonly in association with P. flexuosa, as well as at elevations of 2900m.

The tree is an important source of fuel, construction material, and fodder. The legume is also used as food by local people, particularly in Argentina where pâté made from the pod is still sold in general stores. It is sometimes planted in reforestation projects, but work is needed on the determination of intra-specific genetic variation and the selection of appropriate seed sources for given environmental conditions.

6.5.1 Morphology and anatomy

Stems and roots. Under undisturbed conditions the plan is characteristically a large, single-stemmed tree, 3 to 10 m tall with a well developed crown. The trunk is usually short and the branches knotty and partly spiny. Spines are paired and axillary and some may reach 6 cm in length, but they are not present on all nodes.

Leaves. Leaves are as long or longer than the raceme. The petiole is 1.5 to 12 cm long with 1 to 3 pairs of pinnae 8 to 24.5 cm long which bear 10 to 29 pairs of leaflets. Leaflets are pale green, with poorly visible veins, 10 to 63 mm long, linear and glabrous with entire margins. They are separated on the rachis by 4 to 12 mm, which is equal to or greater than their width of 1.1 to 3 mm.

Inflorescence. Racemes are spikelike, 7 to 12 cm long and bear about 250 flowers. Flower petals are 3 mm long and hairy within. Stamens are 5 to 6 mm long and the ovary is pubescent.

Legume and seed. The legume is linear and flattened, about 12 to 18 cm long, 1 to 1.8 cm broad, and 0.6 cm thick with parallel margins. It is nearly straight to sickle shape and contains 20 to 30 seeds. The mesocarp is sugary. Endocarp segments are rectangular in cross section and broader than long. Seeds are rounded and flattened and are 6 to 7 mm long.

Remarks. This is a highly variable species and has two recognized varieties, var. riojana and var. catamarcana, which are native to Argentive but are not found in Chile or Peru.

6.6 Prosopis flexuosa de Candolle Prodr.

This montane species is found in northern Chile in valleys of the Andes, at elevations of 2,000 m where rainfall is only 50 mm to 100 mm per year. It grows well along the ephimeral streams of the valleys.

This is an important tree for shade, fuel, timber and forage. Large quantities are also cut for the manufacture of tiles for parquet floors.

6.6.1 Morphology and anatomy

Stems and roots. On more favorable sites, the species may become a large, single stemmed tree, but more often it is a many-branched erect shrub 3 to 10 m tall with hemispherical crown. Older trees have drooping, knotty branches with a zig zag appearance. Spines are axillary and paired; they are small and may be absent on older trees, but occasionally reach lengths of 3.4 cm on young, robust shoots.

Leaves. Leaves have 2 to 3 pinnae which are 5 to 13 cm long. The pinnae bear 12 to 29 pairs of leaflets that are spaced at distances usually of several times greater than their width. Leaflets are linear slightly acute to obtuse, 4 to 15 mm long and 1 to 2 mm wide. Their secondary veins may be slightly visible.

Inflorescence. Racemes are spikelike, 4 to 14 cm long, and bear about 200 flowers.

Legume and seed. The pod is nearly straight or arched, 5 to 28 cm long, and 0.7 to 1.2 cm broad with undulating margins (submoniliform). Joints along the pod are nearly square with rounded corners. The color is straw-yellow tinged with violet to nearly black. The pulp is sweet.

Remarks. Burkart (1976 a, 1976 b) states that P. flexuosa is a variable species but well delineated. It differs from P. chilensis in having shorter leaflets and the submoniliform legume, and from P. alba in its more separated and less numerous leaflets and in the submoniliform legume.

6.7 Prosopis glandulosa Torrey

This species is widely distributed in Mexico and extends north-west into the United States. It grows well along drainages in areas where rainfall is less than 150 mm and persists on uplands on neutral and alkaline areas where rainfall is more than 750 mm. It is found at elevations of 1,500 m in areas that have more than 200 frost free days.

The tree is suitable for livestock forage and fuelwood. Also, it is an excellent source of nectar for bees. It is usually considered a weed by ranchers in the United States whose ranges have been invaded by the species and who have mounted extensive eradication programs. Interestingly, cattle are the major factor in propogating the plant on these ranges.

6.7.1 Morphology and anatomy

Stems and roots. The largest individuals that grow in open stands may reach heights of 7 to 13 m. There is also a shrub type that commonly invades grasslands. In dense stands and on sandy sites, it becomes a many-stemmed shrub. Spines are axillary, 1 to 4.5 cm long. They are sometimes paired but usually are solitary. Some individuals have very few spines.

Leaves. Leaves are glabrous and have one or two pairs of pinnae 6 to 17 cm long that bear 6 to 17 pairs of leaflets each. Leaflets are 5 to 12 times as long as wide, mostly 1 to 4 cm long. They are spaced along the rachis at distances as great or greater than their width, and they are linear or oblong, obtuse in shape, and are prominently veined beneath.

Inflorescence. The inflorescence is a spikelike raceme about 5 to 14 cm long. Flower petals are 2.5 to 3.5 mm long and the ovary is hairy.

Legume and seed. The pod is linear, flat, yellow, and about 10 to 20 cm long, 1 cm wide, and 0.5 cm thick. It is either straight or slightly curved. The exocarp is hard and underlaid by a pulpy, sweet mesocarp. Pods contain 5 to 18 oval, glossy brown seeds 5 mm wide, 7 mm long and 2 mm thick.

6.8 Prosopis glandulosa var. torreyana (Benson) Johnston

The distribution, habitat, and use of this variety are similar to those of P. glandulosa. The variety differs mainly in leaf characteristics. The leaflets in P. glandulosa var. torreyana are 15 to 25 mm long, 5 to 8 times as long as wide, and 10 to 15 pairs per pinna. Leaflets of P. glandulosa var. glandulosa are generally 30 to 45 mm long, 8 to 15 times as long as wide, and 6 to 13 pairs per pinna.

6.9 Prosopis juliflora (Swartz) DC

This species is found in northern South America from Peru through Central America into Mexico. It is found throughout Mexico and on the Baja peninsula on flats, lower mountain slopes, and along streams and arroyos.

The legume is sweet and therefore excellent for livestock food. It is also a good source of fuelwood. The species has been introduced for cultivation in Brazil and countries in Asia. It is reported to have become a weed in Haiti, where it has also been introduced.

6.9.1 Morphology and anatomy

Stems and roots. The plant is a small spreading shrub or tree 3 to 8 m tall with a rounded or somewhat flattened crown. The bark is smooth or slightly fissured and brown in color. Spines are axillary and paired, but are sometimes solitary and do not occur on all branches. They vary in length from 0.5 to 5 cm; the longest occur on strong basil shoots.

Leaves. Leaves have one to four pairs of pinnae with 10 to 16 pairs of leaflets. They are shorter or as long as the inflorescence, petiole plus rachid (if present) are 0.5 to 7.5 cm long. Leaflets are elliptical-oblong, 6 to 23 mm long, 1.6 to 5.5 mm wide, and most often glabrous. Along the northwestern coast of South America, the leaflets are occasionally pubescent. They are spaced at distances along the rachis usually a little greater than their width.

Inflorescence. Recemes are roughly cylindrical, 7 to 15 cm long and bear greenish-white flowers that turn light yellow when mature. The calyx is 0.8 to 1 mm long with minute ciliolated teeth. Petals are 2.5 to 3 mm long and hairy within, but glabrous without. Stamens are 4 to 5 mm long.

Legume and seed. The pod is usually flattened and straight but incurves at the apex. On some individuals, it may be sickle-shaped. It is 8 to 29 cm long, 9 to 17 mm wide, and 4 to 8 mm thick. When the pod is immature, the seeds appear as low rounded prominences in the middle of the pod. With age, the pod becomes swollen and pulpy, and the outline of the seeds is no longer discernible. The endocarp may have up to 25 rounded, rectangular segments. Seeds are oval and brown.

Remarks. This species is in the Algarobia group of mesquites, and like other members of the group, it is highly variable. Considerable taxonomic work remains to be accomplished with this group.

6.10 Prosopis juliflora var. horrida (Kunth) Burkart

The tree, native to northern Peru, is found in coastal areas and at elevations above 500 m in some of the most arid areas of the country.

It is valued for shade, timber and fuel. The legume is almost the sole support of livestock in the exceedingly dry areas of Peru where dew is the principal source of moisture. When in flower, it is an excellent source of honey. However, the shrubby form can become an aggressive invader, as it has when it was introduced into Pakistan.

6.10.1 Morphology and anatomy

Stems and roots. This is normally a spiny tree or shrub, although spines on some individuals may be rare. It attains a height of 3 to 8 m. Branches are grayish of black and exhibit a zig zag pattern.

Leaves. Leaves bear 2 to 4 pinnae 6 to 10 cm long. Leaflets are elliptical, ovate to oblong, pubescent, about 15 mm long, and 4 mm wide. Ten to 15 pairs of leaflets are broadly spaced along the rachis.

Inflorescence. Racemes are 9 to 12 cm long. Flowers are greenish/white.

Legume and seed. The legume is straight or arched, compressed, and including the stiple and terminal point, is 18 to 24 cm long and 1 to 1.2 cm broad. It is glabrous, except for scattered hairs along the margins. It has up to 26 rectangular joints that are broader than they are long.

6.11 Prosopis juliflora var. inermis (H.B.K.) Bukart

This variety differs from var. juliflora only in having finely pubescent leaflets and in its lack of spines. However, even with spines lacking, it is not always possible to distinguish from the typical variety.

6.12 Prosopis laevigata (Humboldt and Barpland ex. Willd.)

This species is found primarily in the central highlands of northern Mexico, the lowlands or southern Tamaulipas, and in parts of Oaxaca Morelas, Pueblo, and Chiapas, Mexico. It grows on a variety of sites on hillslides, in depressions, and along flood plains. The species is also found in central and southern Peru.

The tree, often used for shade, is cultivated in the central depression of Chiapas. Legumes are edible and are of value as food for livestock.

6.12.1 Morphology and anatomy

Stems and roots. The tree is usually a chrub of 2 or 3 m but can grow to 6 to 7 m tall. Twigs are glabrous and spines are axillary.

Leaves. There may be one or two pairs of pinnae per leaf, 2.5 to 12 cm long, with 20 to 30 pairs of leaflets each. Leaflets are glabrous, linear, oblong, 5 to 10 mm long, and 2 to 7 times as long as broad. Their interval on the rachis is less than their width; their color is pallid green to grayish, and they are strongly pinnate-veined beneath.

Inflorescence. Racemes are 4 to 10 cm long and bear greenish-white flowers with petals 3 to 4 mm long. The calyx is about 1 mm long.

Legume and seed. Legumes are 9 to 17 cm long and 0,7 to 1.4 cm board. They are linear, glabrous, straight or slightly curved, and pure yellow in color. Segments are rounded or rectangular in cross section and are shorter than they are board. Seeds are longitudinal in the pod.

Remarks. This is highly variable species throughout its range. In Nuevo Leon, individuals are found that are intermediate between P. laevigata and P. glandulosa. Apparent representatives of P. laevigata are found in the general area from southwestern Jalisco to southeastern Nuevo Leon to eastern Durango. The pinnae are usually only one pair per leaf, but there are often two pairs, especially on those trees from Queretaro to Jalisco. Leaflets on these trees are less than 10 mm long, and number 20 to 35 pairs, or even up to 40 pairs per pinna, the highest number counted on any Prosopis in North America.

6.13 Prosopis laevigata var. andicola Burkart

This variety is a montane type and grows at medium elevations of 1900 to 2900 m. It is distributed from southern Peru through the Bolivian Andes to the extreme northwestern corner of Argentina.

Fuelwood and forage are its main uses.

6.13.1 Morphology and anatomy

Stems and roots. These plants are usually shrubs 1 to 3 m tall and more rarely trees 5 to 8 m tall. Twigs are strong and have a pronounced “zig-zag” pattern. Spines are paired and axillary. The most developed spines may be 4 to 9 cm long and 5 to 6 mm broad at the base.

Leaves. Leaves have only one pair of pinnae, 5 to 9 cm long. There are about 16 to 24 pairs of leaflets along the rachis, which nearly touch or overlap.

Inflorescence. Racemes are 6 to 10 cm long, and the florets are glabrous outside.

Legume and seed. The legume is curved, compressed, 7 to 20 cm long, 1.1 to 1.4 cm wide, and rectangular in cross section. Margins are shallowly undulating to parallel, and the exocarp is brown-violet.

6.14 Prosopis pallida (Humboldt and Barpland ex. Willdenow) H. B.K.

This species is native to Peru, Columbia, and Ecuador. It grows in the drier parts of these countries along the Pacific coast.

This species is a valuable shade tree in dry areas. It is used for timber, food and forage. Foresters have introduced it into Hawaii, but they have observed that it can become an invader and form undesirable thickets.

6.14.1 Morphology and anatomy

Stems and roots. The plant can develop into a tree 8 to 20 m high with a trunk of 60 cm in diameter, on good sites. It may remain a shrub on poor sites. Branches are unarmed or have paired, axillary spines less than 4 cm long.

Leaves. Leaves may have 2 to 4 pairs of pinnae, but occasionally only one. The pinnae are 1.5 to 6 cm long, with a cuplike gland at the junction with the petiole. There are 6 to 15 pairs of leaflets per pinna arranged closely together along the rachis, but they do not touch. They are pubescent, oblong to ovate, obtuse, and 2.5 to 8.3 mm long by 1.4 to 4 mm broad.

Inflorescence. Racemes are spikelike and 2 to 3 times longer than the leaves. There are 200 to 250 greenish-yellow flowers per raceme. The flower petals are separate, 2.5 to 3 cm long, and hairy within. The ovary is hairy, and the calyx is ciliolated and 0.5 to 1.5 mm long.

Legume and seed. The legume is straight or curved, straw-yellow, has parallel margins, is sometimes subquadrate-rectangular in cross section, and is 10 to 25 cm long, 1 to 1.5 cm broad, and 5 to 9 mm thick. Segments of the legume are broader than they are long. There are up to 30 oblong, brown seeds, 6.5 mm long in each pod.

Remarks. Burkart (1976 a, 1976 b) could find no consistent differences between P. pallida and P. limensis and placed the latter name in synomym with the former. He states that both species, that were described originally, were from Peru but that the author of P. limensis apparently did not compare it with P. pallida which had been classified earlier.

6.15 Prosopis palmeri S. Wats

This species of Prosopis is narrowly restricted primarily to the peninsula of Baja California. It grows mainly in or near dry stream beds, in canyons, and on desert playas.

The plant is much used for fuel and browse for livestock. It also supplies the material for the small “ironwood” carving industry of the Seri Indians.

6.15.1 Morphology and anatomy

Stems and roots. The shrub or tree is many-branched, spreads symmetrically and is flat-topped. It varies in height 1.2 to 6 m. The trunk may be 1 to 3 m long and reach 45 cm in diameter. The bark is gray or dark purple, splitting off in long strips. Branches are spiny, finely pubescent, or somewhat glabrous. The spiny stipules are usually straight and paired and are 0.5 to 3.5 cm long, but they shear off with age. The dark purple wood is very hard from which the plant derives its common name, “Palo de Hierro”.

Leaves. Leaves tend to cluster on short shoots along the branch. They have a single pair of pinnae 2 to 3 cm long. Leaflets are in 3 to 10 pairs along the pinnae. They are glabrous and elliptical-oblong, obtuse or micronate, tending to obovate.

Inflorescence. Racemes are axillary, slender spikes 3.5 to 6 cm long. Flowers are minutely downy and tubular. The calyx is 1.3 mm long, and the corolla 5 toothed and 4.5 to 6 mm long. Petals are joined for much of their length and are nearly glabrous inside the tips. Stamens are 10 to 12 mm long, and the anthers are 0.8 to 1.5 mm long. The ovary is white and hairy.

Legume and seed. Legumes are straight to sickle-shape, indehiscent, minutely downy, and almost rounded to compressed. They are 4 to 9 cm long and from 0.8 to 1 cm wide. The exocarp is thin, brittle, and nearly membranous. The mesocarp is thin, brittle, tubular, and bright lemon-yellow in color. Segments are rectangular. Seeds are numerous and arranged obliquely. They are oblong, brown, smooth, 5 to 7 mm long, and more or less separated by a thick grummy, reddish-brown substance.

6.16 Prosopis pubescens Bentham

This species is found in northern Mexico on the peninsula of Baja California and in the states of Chihuahua and Sonora. It grows along stream banks, on bottom-lands, and about water holes.

The tree is used as fuel, and the legumes are important as forage for cattle.

6.16.1 Morphology and anatomy

Stems and roots. The plant is an openly-branching tree or shrub, 2 to 10 m tall. The twigs are slender, and the spines are stipular arising from beneath the leaf cluster. Trunks are short with thin, flaky bark, and reach diameters of 30 to 40 cm.

Leaves. Leaf petioles are short and slender, 0.5 to 2 cm long. Pinnae occur in one or 2 pairs and are 2 to 3 cm long. Leaflets are elliptical-oblong or oblong-obtuse, 2 to 5 cm wide, and 6 to 10 mm long. They do not overlap, occur on the pinnae in 4 to 9 pairs, are finely pubescent to glabrous, and obscurely veined.

Inflorescence. The dense spikelike racemes are axillary, simple, and 5 to 8 cm long. Florets are pubescent and yellow in color. The calyx is broadly bowl-shaped, 0.7 to 1.2 mm long, and has minute teeth. Petals are 2.5 to 3 mm long, glabrous within, and united. Petals are finely pubescent outside and on the inner apex. Stamens are 4 to 5 mm long, and the anthers 0.5 to 0.8 mm long. The ovary is white and hairy, and the style is glabrous.

Legume and seed. Pods are sessile, tightly coiled, and spiral. Young pods that are only 4 to 5 mm long may have already developed two to three coils. The mature fruit has 8 to 24 dense, closely-set spirals. Fruits have a cylindrical shape 2.5 to 5.5 cm in diameter, commonly called “screw-bean” or “tornillo”. Seeds are kidney-shaped, minutely downy, and 1.5 to 2 mm long.

6.17 Prosopis reptans Bentham

This species is found on arid sites in Peru and central Argentina.

Its uses are unknown, but the plant likely serves as forage for livestock.

6.17.1 Morphology and anatomy

Stems and roots. The species is commonly a spiny desert shrub 1 to 3 m tall, and more rarely a tree 5 to 8 m tall. It produces underground runners by which it invades an area and becomes a thicket. Twigs are strong and have a pronounced zig-zag pattern. Spines are axillary and occur in pairs; the most developed attain lengths of 4 to 9 cm and widths at the base of 5 to 6 mm.

Leaves. Pinnae are 5 to 9 cm long, occur in a single pair, and have 16 to 24 pairs of leaflets. Leaflets are leathery, linear-oblong, 5 to 12 mm long, and 1.5 to 2.2 mm wide. They nearly touch or are spaced at distances less than their widths along the rachis.

Most of the characteristics of the species are very similar to those of P. stombulifera, with the exception of the leaves. The leaflets are smaller, more pubescent, and with margins touching each other in P. stombulifera.

Inflorescence. Flowering heads are axillary and globular. The corolla is striated. The inflorescence is a round ball.

Legume and seed. Pods develop into numerous, close, regular spirals (up to 19), forming a cylindrical body about 5 cm long and 7 mm in diameter. Spirals are held in a radiated position and do not intermingle.

6.18 Prosopis strombulifera (Lamb.) Bentham

This species is found in western Argentina, northern Chile, and Peru. It grows in arid areas and is adaptable to the fringes of saline and alkaline areas.

The fruit is astringent and diuretic. It has been collected and sold in markets as a remedy for a number of ills, including venereal disease. In the last century, it was reported that the plant was cultivated in Chile to be used as a remedy for toothaches, and it has been suggested that its curious shape lends itself to imaginary cures. The roots contain a brown dye and may be used for tanning.

6.18.1 Morphology and anatomy

Stems and roots. The plant is a spiny shrub 0.15 to 1.5 m tall, with underground spreading runners. Branches develop in a zig zag fashion. Spines are straight, ash colored stipules, 0.1 to 2 cm long, which point sharply downward.

Leaves. Leaves have a single pair of pinnae 1 to 3 cm long, with 3 to 8 pairs of leaflets per pinna. They are somewhat waxy, pubescent, or glabrous, 2 to 15 mm long, 0.8 to 2 mm broad, oblong, and one to three-nerved at their base. Leaflets are spaced along the rachis at distances one to three times their width.

Inflorescence. Inflorescences are acillary, solitary, peduncle 1.5 to 5 cm long. Heads are globose, about 1.5 cm in diameter and support yellow flowers. Both calyx and corolla are striated. The calyx is downy, 1.5 to 2.2 mm long. Petals are united the distance of their width. They are 3 to 4 mm long and hairy within. The stamen and style extend above the corolla, and the ovary is hairy.

Legume and seed. Legumes are lemon-yellow when mature and downy when young. They are tight coiled into a dense, closed corkscrew like cylinder, 3 to 5 cm thick. The exocarp is brittle. The endocarp is segmented and easy to open. The mesocarp is pulpy, tannic, and reddish in color. Seeds are ovate, grayish-green, and 4.5 to 5.4 mm long.

6.19 Prosopis tamarugo F. Philippi

This species is found in northern Chile, only in the desert Provinces. In the Province of Tarapaca, where on arid mesas and bajadas it is the only tree, it apparently grows under essentially rainless conditions. Its area of distribution covers about 15,000 km2 in the central strip of Tarapaca, south of Arica and Antofagasta, and north of Chuquimata. It occurs in scattered stands which very often have been devasted for fuelwood. The tree grows well on saline soils.

The species is a major, and quite often the only, source of fuel wood in its area of distribution. It is valuable for charcoal, wood for poles, rustic construction and furniture, and is a source of honey. It is presently being used to reforest large areas of desert. The tree is reported to be especially valuable for raising Angora goats.

6.19.1 Morphology and anatomy

Stems and roots. The tree may reach heights of 18 m a trunk of one meter in diameter. Spines are formed by hardened stipules, 0.5 to 3.8 cm long, and yellow in color. Roots of the species do not necessarily tap the water table. Instead, the tree appears to derive all of its moisture from the atmosphere.

Leaves. The petiole is 1 to 3 mm long and supports a single pair of pinnae 3 to 4 cm long. There are 10 to 15 pairs of leaflets per pinna. They are linear and obtuse or subacute, 4 to 8 mm long, and glabrous.

Inflorescence. The inflorescence is spikelike and 3.5 to 6.5 cm long. Flowers are sessile, glabrous, and yellow in color. Petals are united and have hairy tips. The corolla is 4 to 5 mm long, and the calyx is 1.5 mm long. The ovary is hairy.

Legume and seed. The legume is cylindrical with somewhat swollen portions at intervals, strongly incurved almost to a ring shape but not coiled. The ring is 2 to 3.5 cm in diameter and 7 to 10 mm thick. Seeds are ovate, 3 to 4.3 mm long, and brown.

Remarks. Many of the characteristics of this species are similar to those of P. ferox, whose distribution is limited to southern Bolivia and the extreme northwest of Argentina.

6.20 Prosopis tamaulipana Burkart

This species is found in Mexico, east of the Sierra Madre Oriental, primarily in states of Tamaulipas and Vera Cruz. Many of the specimens collected in herbariums were found growing on heavy clay soils at low elevations. Its use is unknown.

6.20.1 Morphology and anatomy

Stems and roots. Trees are small to medium size, with rounded crowns and short trunks, and with rough and gummy bark. Branches are slender, knotty, green, and droop as do willows. Spines are axillary, 0.5 to 1.5 cm long, and are not present on all nodes. Leaves are mostly fasticular with one to three pairs of pinnae, 2.5 to 4 cm long. There are 15 to 29 pairs of leaflets per pinna. They are oblong, obtuse, grayish-green, 2.8 to 6 mm long, 1 to 1.8 mm wide, and glabrous to slightly downy.

Inflorescence. Racemes are 5 to 8 cm long, and bear small greenish-white flowers with petals 3 to 3.5 mm long. The calyx is 1 to 1.5 mm long. Stamens are 5 to 6 mm long, and the ovary is hairy.

Legume and seed. The legume is straight, 8 to 13 cm long, and 6 to 8 mm broad. It is usually glabrous, and straw-yellow with longitudinal striae. Margins are somewhat constricted between the seeds. The mesocarp is pulpy and sweet. There are 10 to 18 hard, ovate to slightly square, obliquely-oriented endocarp segments, 5.5 mm long, 6 to 6.5 mm wide, and 3.5 mm thick. Seeds are ovate, brown, and 5 to 5.5 mm long.

6.21 Prosopis velutina Wooten

This important species is found in northwestern Mexico, in the State of Sonora. It grows in flood plains, in low areas of playas, and along desert wash systems.

This plant, like most of the Prosopis species in southwestern North America was, and to a more limited extent, still is, an important resource to the native people of the region. It has been used for food, fuel, shelter, tools, dye, fiber, cosmetics, medicine, and a multitude of other practical and aesthetic purposes.

6.21.1 Morphology and anatomy

Stems and roots. Under undisturbed conditions, the plant is single-stemmed due to a strong tendency for apical dominance. Individual trees may obtain heights of up to 18 m. However, the apical meristem is easily damaged by grazing animals, insects, or physical processes such as freezing. Consequently, the plants are more commonly multi-stemmed and shrubby. Roots of P. velutina have been excavated at a depth of almost 50 m and over horizontal distances of more than 20 m. Therefore, the plant is efficient in utilizing the available water in the upper soil layers, as well as that in the deeper layers, which in some cases may lie below the water table.

Leaves. Leaves, like all species of the genus, are doubly compound. Usually two pair of pinnae, but sometimes one or three pairs, may be present. The pinnae are 2 to 9 cm long and may support 12 to 30 pairs of leaflets. Leaflets almost or actually touch along the rachis. They are oblong and pubescent with entire margins. Stomata are found on the rachises and on both surfaces of the leaflets. They are not sunken nor protected by trichomes, and, therefore, appear more like those of mesophytes. The leaf surfaces exhibit single-celled glandular trichomes and wax crystals, which produce a velvety texture from which the species dervies its common name, velvet mesquite.

Inflorescence. Racemes are spikelike 5 to 15 cm long with pale greenish-white florets. Florets have short pedicells and are somewhat pubescent on the outside, especially on the calyx. Petals are about 2.5 mm long with tips that are hairy on the inside. The principal bloom occurs during the spring, but it normally follows a winter rainy season. Later in the year, a second bloom, of typically lower intensity than the first, may occur. A small, multi-branched plant may produce about 6,000 inflorescences during a year.

Legume and seed. The legume is linear and quite straight, 8 to 16 cm long and 6 to 10 mm broad. Joints of the pod are about as long as broad or shorter.

Fruit color can vary from pure yellow to mottled red or black. The outer surface is velvety on immature pods but becomes smooth at maturity. Protein content of the pod is about 12 percent, and sugar content is 16 percent. Seeds, as are characteristic of Prosopis, are contained within a hard and stony endocarp.

Remarks. When they first germinate, seedlings of P. velutina, like most Prosopis species, have a prominent tap root and a pair of oval, somewhat fleshy cotyledons. Root growth has been observed to be ten times greater than shoot growth, which is attributed to an adjustment to water strees rather than to any genetically determined tendency for precocious root growth. Germination is hampered and seedling mortality is high in areas of established grass cover.

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