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).