Passiflora tenuiloba, commonly known as the birdwing passionflower or spread-lobe passionflower, is a species of flowering plant in the family Passifloraceae. It is a slender herbaceous vine native to the south-central and southwestern United States and northeastern Mexico. It occurs primarily in open, rocky limestone habitats in the U.S. states of Texas and New Mexico, and in the Mexican states of Coahuila, Nuevo León, San Luis Potosí, and Tamaulipas. The species is distinguished by its highly variable, usually three-lobed leaves. On the larger leaves, the primary lobes are frequently divided into additional narrow lobes, producing the wing-like outline reflected in the common name birdwing passionflower.
Description Passiflora tenuiloba is a perennial, herbaceous or somewhat woody vine that climbs or trails by means of axillary tendrils. Its stems are slender and branching. The leaves are alternately arranged and vary considerably in shape on an individual plant. Most mature leaves are broadly three-lobed, with the lateral lobes spreading nearly horizontally. The lobes of larger leaves are often divided again into narrower secondary lobes. This gives the leaf blade a deeply cut, birdwing-like appearance. Unlobed or irregularly lobed leaves may also occur, especially on younger growth. The flowers are small and borne singly from the leaf axils. As in other members of the subgenus Decaloba, the flowers lack true petals, so the visible perianth consists of five greenish to pale-colored sepals surrounding a filamentous corona. The central reproductive column bears five stamens and a three-branched style. The fruits are small, rounded to ellipsoid berries that become dark blue, purple, or nearly black when mature. They contain numerous seeds surrounded by fleshy arils. Flowering and fruiting may occur from April through October, and sometimes into December in areas with mild winters.
Taxonomy Passiflora tenuiloba was formally described in 1850 in the Boston Journal of Natural History. The description was published by Asa Gray, who attributed the name and diagnosis to German-American botanist George Engelmann. The original material was collected by German botanist Ferdinand Jacob Lindheimer in October along the Llano River in Texas. The name was published as:
Passiflora tenuiloba Engelm., Boston Journal of Natural History 6(2): 192. 1850. The specific epithet tenuiloba is derived from Latin words meaning "slender-lobed" or "narrow-lobed", referring to the plant's thin, deeply divided leaf lobes. Passiflora bigelovii, described by John Kunkel Small, has sometimes been treated as a separate species but is now generally included within P. tenuiloba.
Classification Passiflora tenuiloba belongs to the subgenus Decaloba, a group characterized by relatively small flowers and generally small, dark-colored fruits. Within the southeastern United States, it is most readily confused with Passiflora pallida. The two species differ principally in leaf shape. The leaves of P. pallida may be unlobed or divided into two or three entire lobes, with the lateral lobes ascending at an angle of less than 150 degrees. In P. tenuiloba, the mature leaves are typically three-lobed, the larger lobes are often further subdivided, and the lateral lobes spread at approximately 180 degrees from one another. It differs from Passiflora lutea and Passiflora affinis in lacking petals. Those species have flowers with both five sepals and five petals, whereas the perianth of P. tenuiloba consists of five sepals only.
Distribution Passiflora tenuiloba is native to the United States and northeastern Mexico. In the United States it occurs primarily in central, eastern, and southern Texas, with additional records from New Mexico. Its Mexican distribution includes Coahuila, Nuevo León, San Luis Potosí, and Tamaulipas. The range places the species chiefly within and near the Edwards Plateau, the southern Great Plains, the Tamaulipan mezquital, and the northeastern margin of the Chihuahuan Desert.
Habitat and ecology The species grows in exposed limestone habitats, including rocky slopes, limestone ledges, open calcareous shrublands, woodland openings, and rocky roadsides. Its high heliophily rating indicates that it is strongly associated with open, sunny conditions rather than densely shaded forest. Like other passionflowers, P. tenuiloba is likely pollinated by insects attracted to its floral corona and nectar. Its leaves may also serve as food for the larvae of butterflies in the tribe Heliconiini, although species-specific host relationships require further documentation.
Conservation NatureServe assigns Passiflora tenuiloba a global conservation rank of G5, meaning globally secure. However, that rank was last reviewed on 10 December 1992 and is identified by NatureServe as needing review. Within the United States, NatureServe lists the species as unranked nationally. It is also unranked in both New Mexico and Texas. It has no status under the United States Endangered Species Act.
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