2. Apocynum L. (dogbane, Indian hemp)

Plants perennial herbs, sometimes somewhat woody at the base. Stems solitary or few (but plants sometimes forming large colonies from elongate, branching rhizomes), erect or ascending, often branched dichotomously (usually above the midpoint), glabrous. Leaves opposite or sometimes some of them subopposite or alternate, short-petiolate or sessile. Leaf blades ovate to narrowly lanceolate, glabrous or pubescent with nonglandular hairs, rounded to angled and often with a minute, abrupt, sharp point at the tip, often somewhat asymmetrical at the angled to rounded or less commonly shallowly cordate base. Inflorescences terminal or axillary, branched loose clusters of few to many flowers. Flowers sweetly fragrant. Calyx lobes sharply pointed at the tip, glabrous or with nonglandular hairs. Corollas bell-shaped to urn-shaped, bearing usually 5 small, scalelike appendages internally at the base, white to sometimes tinged or lined with pink, mostly glabrous, the lobes less than half as long as the tube, erect to spreading. Stamens attached near the base of the corolla tube, incurved to form a cone over the stigma, the anthers adhering to the stigma by sticky secretions; anthers arrowhead-shaped with short, triangular, basal lobes. Nectar glands 5, positioned around the ovary bases alternating with the stamens. Styles very short, the stigma more or less ovoid, encircled by a narrow thickening or rim around the midpoint. Fruits slender, elongate. Seeds numerous, narrowly cylindrical, somewhat tapered toward the base, with a tuft of hairs at the truncate tip. About seven species, North America.

Estimates of the number of species of Apocynum range from 2 to more than 80, depending on whether Old World taxa are included, how hybrids are treated, and what characters are emphasized. Following Woodson’s (1930) treatment, most authors recognize about 7 species that are native to North America, with the Old World species consigned to such segregate genera as Trachomitum Woodson and Poacynum Baill. In general, characters of size, shape, and pubescence of the leaves are not considered reliable. Woodson emphasized the angle at which the leaves are held (a character that unfortunately is often difficult to see in pressed specimens), the relative length of the calyx lobes and corolla tube, and the size and orientation of the follicles. Variation in flower color from pink to white with various markings also has been noted, but this also is lost frequently when specimens are pressed and dried. In actual practice, it is quite difficult to separate the species, and there are many specimens that are intermediate between the extremes of variation. The most reliable character for separating the species appears to be the length of the corolla tube relative to the calyx lobes.

The pollination biology of Apocynum remains something of a mystery. Production of follicles is relatively uncommon despite a high visitation rate by insects. Woodson (1930) suggested the low fruit set indicated that the flowers were self-incompatible. Lipow and Wyatt (1999) found that self-pollinated flowers of A. cannabinum produced no fruit, and they concluded that this species was indeed self-incompatible. There appears to be a failure of early embryo development in self-pollinated flowers, and the physical separation of the anthers from the receptive lateral portion of the stigma prevents direct transfer of pollen as the anthers dehisce. The flowers produce nectar and attract a wide assortment of flies, bees, and butterflies, but rarely do the insects affect pollination. Johnson et al. (1998) investigated reproductive biology and hybridization in A. ×floribundum and its presumed parental species in Colorado. Caged plants from which pollinators were excluded failed to produce any follicles. Fruit set was very low in populations of the uncaged parental species and was almost nonexistent in the uncaged hybrids. Insect visitors were found to be carrying little or no Apocynum pollen, although Waddington (1976) had earlier found A. sibiricum pollen on the proboscis of butterfly visitors. On the other hand, population genetic studies involving allozyme variation (Johnson et al., 1998) indicated that the hybrid populations are more heterozygous than the parental species, which had low levels of heterozygosity, suggesting a long history of inbreeding. Populations of all of the species often consist of one or few large clones, with few unique genotypes.

Apocynum has had a long history of use by humans. Fibers derived from the plants have great strength and formerly were used as a source of thread and cord for making fishnets, bags, and clothing. The fibers can also be used to make paper. The latex can be used to produce a type of rubber and also contains powerful alkaloids that stimulate the heart much like digitalis. All of the species produce abundant nectar and are considered good bee plants. However, some species, such as A. cannabinum, can be troublesome weeds in crop fields, pastures, and gardens, and may have allelopathic effects on cultivated plants. Yield reductions of up to 45 percent have been reported from infested crop fields in Kansas (McGregor, 1984).