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Bull-horn. A name applied to several species of tropical American acacias remarkable for their large stipular inflated spines which closely resemble the horns of an ox or buffalo. These are utilized by certain stinging ante of the genus Pseudomyrma as nesting-places for rearing their young. The thorns, which are connate at the base, are hollowed put by the insects, which perforate one of the spines near the tip, usually on the under side, so that no water can enter. All the species of true bull-horns have a four-lobed involucel on the peduncle of the flower-spike near the base. The bipinnate leaves have nectar-glands on the rachis and petiole, as in many other acacias, and they are still further provided with peculiar processes on the tips of the leaflets, minute wax-like bodies rich in oil and protoplasm, which Thomas Belt, in his "Naturalist in Nicaragua" (1874), discovered to be used as food by the ants inhabiting the spines, and which in his honor were named Beltian bodies. These apical bodies had long been known, and Linnreus called attention to the nectaries on the leaf-rachis, but Belt was the first to suggest that in return for quarters and subsistence the little ants serve their host as a body-guard of soldiers, and Darwin in his work on the "Effects of Cross- and Self-Fertilization in the Vegetable Kingdom," called attention to Belt's interesting observations and deductions.

Francisco Hernandez, the protomedico of Philip II of Spain, sent in 1570 to study the resources of Mexico, figured the peculiar spines and the leaves of one species growing in the Huasteca region of Mexico, in the Tierra- caliente, not far from the Gulf coast. This author speaks of the intense pain caused by the stings of the ants and describes their laryse engendered in the hollow spines. Jacquin, in describing a bull-horn acacia growing near Cartagena (Colombia) in 1763, tells how the little insects rush from the thorns when the tree is struck however lightly, falling upon the unwary intruder and inflicting upon him myriads of burning stings. Long before this (1696) Plukenet had figured the bodies on the apices of the leaflets, and Linnaeus himself expressed his wonder as to the function of the extra-floral nectar glands.

In all bull-horn acacias, there are two kinds of leaves with accompanying spines: vegetative leaves in which the stipular spines usually become greatly inflated; and bract-Like smaller 694. Acacia cornigera. leaves subtending the flower-heads or flower-spikes on the axillary raceme-like flowering branchlets, with stipular spines usually small and subulate. The extra-floral glands on the leaf-rachis and petiole are either crater-like and more or less elongated, or round and bead-Like, often several in a series at the base of the petiole and sometimes one between each pair of pinnae.

The flower-spikes or flower-heads are solitary, geminate, or fascicled in clusters of several in the axils of the small bipinnate leaves on the axillary, raceme-like flowering branchlets. In one species, Acacia Cookii, there is apparently no specialized flowering branchlet, but the globose heads are borne in dense clusters in the axils of the large slender-pronged equitant spines. In all true bull-horns the four-toothed involucel is at or near the base of the peduncle. In A. cochliacantha the involucel is at the apex of the peduncle, very much as in A. Farnesiana. In A, cornigera, A. spadicigera. and A. Collinsii, the spikes are dense, cylindrical and more or less like the spadix of an aroid. In A. sphsero- cephala they are sphaeroid-ovate or ovate-oblong, with the flowers closely crowded on a fuslform receptacle. In A. Cookii, the heads are perfectly globose with the receptacle also globose. In A. Hindsii, which Bentham put in a section (Americans laxlflorse) apart from .4. spadicigera and its allies {Pycnanlhs americanse), the flower-spikes are lax and slender with flowers not very closely crowded.

Between the small flowers are stipitate bracteoles or umbracula which may readily be Likened to minute umbrellas with slender handles protecting the flowers before anthesis from' moisture and fungus spores. The laminae of these may be ovate-acuminate or hastate and long- pointed, as in A. cornigera and A. spadicigera; ovate with the margin ciliate, as in A. sphserocephala; circular or nearly so, as in A. Collinsii and A. Hindsii, or very broadly ovate, as in A. Cookii. The flowers themselves consist of a tubular calyx, four- or five-toothed or almost entire, a corolla of four or five lobes, in A. cornigera and its allies only slightly longer than the calyx but in A. Hindsii about twice as long. They are polygamous; that is, some of the flowers are entirely staminate, others are both stami- nate and pistillate. The stamens are numerous, with a single pistil in the hermaphrodite flowers rising from the center of the mass; ovary several-ovuled; style fillform, stigma minute, terminal.

In one division, to which A. cornigera and its allies belong, the pods are indehiscent, inflated, thin, char- taceous, terminating in a sharp beak (Fig. 693). In another division, to which A. Hindsii and A. Collinsii belong, the pods are dehiscent (Fig. 696). In A. Cookii they are very long and slender and two-valved. In all cases the hard smooth compressed seeds are surrounded by sweetish yellow or orange-colored pulp, somewhat like that found in the pods of the algarroba, or St. John's bread, which causes the fallen pods to be eagerly sought by pigs and other animals. This peculiarity at once distinguishes the bull-horn acacias from A. arabica, the type of the genus, which has dehiscent pods devoid of pulp.

Following are the leading species of bull-horn acacias:

A. cornigera. Linn. (Arbor cornigera, Hernandez). Hoitt- Hamaxalli. Bull-horn. Cuernitos. Arbol De Las Hormioas. Ant-tree. Figs. 693,694. A shrub or small tree with 1-3 erect sta. and a few lateral branches bearing numerous large inflated spines remarkable for their close resemblance to the horns of an ox or buffalo. The pods are eaten by pigs and other animals. E. Trop. Mex.

A. nicoyensis, Schenck. Nicota Bull-horn. Espino Blanco. White-spined Bull-horn. A shrub or small tree resembling the former. Occurs in Costa Rica, Guatemala and the adjacent regions of Mex. and Salvador.

A. sphterocephala, Cham. & Schl. Bull-horn. Cuernitos De Veracruz. Cornizuelo. Arbol De Las Hormigas. Arbol Hormiquero. Fig. 695. A shrub or small tree resembling A. cornigera, from the state of Vera Cruz.

A. Cookii, Safford. Bull-horn Acacia Op Alta Verapaz. A small tree or shrub with slender fork-like inflated thorns inhabited by stinging ants. Guatemala.

A. Collinsii. Safford. A shrub or small tree with inflated I - shaped, olive-greenish or brownuh stipular spines curving upward, and sometimes twisted around the branch. S. Mex. DD. Spike lax, flexible, linear: pods coriaceous, slender, falcate usually long-acuminate: larger spines very broad and flat, terminating in widely diverging, very sharp points, like an inverted bicorn chapeau.

A. Hindsii, Benth. Broad-thorn Acacia. Boll-thorn Op Mazanillo. Buffalo Horn. Fig. 69H. A small glabrous tree remarkable for its broad flat stipular thorns, which resemble in form on inverted military chapeau. W. coast of Mex.

A. cochtiacantha, H. & B. (Mimoaa campeachiana, Miller). Split-thorn Acacia. Spoon-thorn Acacia. This species ia not a true bull-horn since its peduncles have not a basal in- yolucel and its atipular spines instead of being hollow and subject to the perforations of ants become split longitudinally.
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==Gallery==

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==References==
<references/>
*[[Standard Cyclopedia of Horticulture]], by L. H. Bailey, MacMillan Co., 1963
<!--- xxxxx  *Flora: The Gardener's Bible, by Sean Hogan. Global Book Publishing, 2003. ISBN 0881925381  -->
<!--- xxxxx  *American Horticultural Society: A-Z Encyclopedia of Garden Plants, by Christopher Brickell, Judith D. Zuk. 1996. ISBN 0789419432  -->
<!--- xxxxx  *Sunset National Garden Book. Sunset Books, Inc., 1997. ISBN 0376038608  -->

==External links==
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