50S 



BLLLACE 



BULL-HORN 



693. Acacia comigera; 



spoken of as bullaoes) are usually lofoni'il to I he hot aui- 

 cal ntmie of Frwius ifisitUin (e.g., Hedriok, Plums of 

 New York, p. 40); but they are also classified with the 

 Danisons, thus taking the botauioal njime of Pruuus 



doincslica var. damas- 

 ccna (See Bot. Gaz. 

 27:481.) 



F. A. Waugh. 



BULL-HORN. A 



name apjilied to sev- 

 eral species of tropical 

 American acacias re- 

 markable for their 

 large stipular inflated 

 spines which closely 

 resemble the horns of 

 an ox or buffalo. These 

 are utilized by certain 

 stinging ants of the 

 genus Pseudomyrma 

 as nesting-places for 

 rearing their young. 

 The thorns, which are 

 connate at the base, 

 are hollowed out by 

 the insects, which per- 

 forate 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 

 bipinnatc 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 wa.x-like bodies rich in oil and 

 protopla.sm, 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 Linnsus 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 Cro.s.s- 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 1.570 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 larva; engendered in the hollow 

 spines. Jacquin, in describing a bull-horn acacia grow- 

 ing near Cartagena (Colombia) in 1763, tells how the 

 little insects j-ush from the 

 thorns when the tree is struck 

 however lightly, falling upon 

 the unwary intruder and infiiot- 

 ing upon him myriads of burn- 

 ing stings. I.ong before this 

 (\()U()) Plukenct ha<l figured the 

 bodies on the apices of the leaf- 

 lets, and Linna>us himself ex- 

 ))reKsed his wonder as to the 

 function of the extra-floral nectar 

 glands. 



In all bull-horn acacias, there 

 arc 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 

 antl subulate. The extra-floral glands on the leaf-rachis 

 and [letiole are either crater-like and more or less 

 elongated, or roimd and bead-like, often several in a 

 series at the base of the petiole and sometimes one 

 between each pair of pinna-. 



The flower-spikes or flower-heads are .solitary, gemi- 

 nate, or fascich^d in clusters of several in the axils of 

 the small bipinnatc 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. cochliacanlha 

 the involucel is at the apex of the peduncle, very much 

 as in ,4. Famesiana. In A. cojrnigera, A. spadicigera, 

 and A. CoUinsii, the spikes are dense, cylindrical and 

 more or less like the spadix of an aroid. In .4. sphsero- 

 cephala they are sphicroid-ovate or ovate-oblong, with 

 the flowers closely crowded on a fusiform receptacle. 

 In A. Cookii, the heads are perfectly globose with the 

 receptacle also globose. In A. Hindsii, which Bentham 

 put in a section (Americanx laxiflorx) apart from .4. 

 spadicigera and its allies 

 {Pycnanlhx american3e), the 

 flower-spikes are lax and slen- 

 der with flowers not very 

 clo-sely crowded. 



Between the small flowers 

 are stipitate bracteoles 

 umbracula which may 

 readily be hkened to minute 

 umbrellas with slender 

 handles protecting 

 the flowers before 

 anthesis from' 

 moisture and fun- 

 gus spores. The 

 lamina; of these 

 may be ovate-acu- 

 minate or hastate and long- 

 pointed, as in A. condgera 

 and A. spadicigera; ovate 

 with the margin ciliate, as in 

 A. sphserocephala; circular or 

 nearly so, as in A. CoUinsii 

 and A. Hindsii, or very 

 broadly ovate, as in ^ . 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. comigera 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 

 filiform, stigma minute, terminal. 



In one division, to which A. cornigera and its allies 

 belong, the pods are indehisccnt, inflated, thin, char- 

 taceous, terminating in a sharp beak (Fig. 693). In 

 another division, to which A. Hindsii and A. CoUinsii 

 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 sur- 

 rounded 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. nrahica, the type of the genus, which has 

 dehiscent pods devoid of pulp. 



Following are the leading species of bull-horn 



6Q5. Acacia sphserocephala. 



