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Asclepias
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Asclepias
Asclepias is a genus of herbaceous, perennial, flowering plants known as milkweeds, named for their latex, a milky substance containing cardiac glycosides termed cardenolides, exuded where cells are damaged. Most species are toxic to humans and many other species, primarily due to the presence of cardenolides. However, as with many such plants, some species feed upon milkweed leaves or the nectar from their flowers. A noteworthy feeder on milkweeds is the monarch butterfly, which uses and requires certain milkweeds as host plants for its larvae.
The Asclepias genus contains over 200 species distributed broadly across Africa, North America, and South America. It previously belonged to the family Asclepiadaceae, which is now classified as the subfamily Asclepiadoideae of the dogbane family, Apocynaceae.
In the first century AD, Dioscorides described a plant called ἀσκληπιάς (asklepias) in Greek and Pliny the Elder described a plant called asclepias in Latin. Though usually explained as being derived from the name of the Greek medical god Asklepios, neither author mentioned the god in connection with this plant. Their plant has since been commonly identified as the plant now known as Vincetoxicum hirundinaria. Some botanists have disagreed as, though the old descriptions are short and vague, Vincetoxicum hirundinaria does not have leaves like ivy.
Linnaeus first described the genus Asclepias in his Genera Plantarum of 1737 without specifying any particular plants in the genus. Linnaeus published his Flora Suecica in 1745 and Materia Medica in 1749. The only Asclepias he mentioned in either book was the one he would later name as Asclepias vincetoxicum. Both books mentioned that in pharmacy this plant was called Hirundinariæ. In none of his works does Linnaeus give an origin for the name Asclepias, presumably because it was being used for these plants by some herbalists and botanists for centuries before. These publications were before his 1753 innovation of names that were simply two words - genus and species - rather than a much longer description.
When Linnaeus described the 18 species that he assigned to the genus Asclepias in his Species Plantarum of 1753, he included Asclepias vincetoxicum. That plant is now called Vincetoxicum hirundinaria. Of those 18, 7 have been assigned to other genera in the years since: Xysmalobium undulatum, Calotropis gigantea, Gymnema lactiferum, Vincetoxicum hirundinaria, Vincetoxicum nigrum, Gomphocarpus fruticosus and Cynanchum thesioides. The species that remain in Asclepias from those Linnaeus described are: A. syriaca, A. amoena and A. purpurascens both now regarded as A. purpurascens, A. variegata, A. nivea, A. incarnata, A. curassavica, A. decumbens and A. tuberosa both now regarded as A. tuberosa, A. verticillata and A. rubra.
Members of the genus produce some of the most complex flowers in the plant kingdom, comparable to orchids in complexity. Five petals reflex backwards revealing a gynostegium surrounded by a five-membrane corona. The corona is composed of a five-paired hood-and-horn structure with the hood acting as a sheath for the inner horn. Glands holding pollinia are found between the hoods. The size, shape and color of the horns and hoods are often important identifying characteristics for species in the genus Asclepias.
Pollination in this genus is accomplished in an unusual manner. Pollen is grouped into complex structures called pollinia (or "pollen sacs"), rather than being individual grains or tetrads, as is typical for most plants. The feet or mouthparts of flower-visiting insects, such as bees, wasps, and butterflies, slip into one of the five slits in each flower formed by adjacent anthers. The bases of the pollinia then mechanically attach to the insect, so that a pair of pollen sacs can be pulled free when the pollinator flies off, assuming the insect is large enough to produce the necessary pulling force (if not, the insect may become trapped and die). Pollination is effected by the reverse procedure, in which one of the pollinia becomes trapped within the anther slit. Large-bodied hymenopterans (bees, wasps) are the most common and best pollinators, accounting for over 50% of all Asclepias pollination, whereas monarch butterflies are poor pollinators of milkweed.
Asclepias species produce their seeds in pods termed follicles. The seeds, which are arranged in overlapping rows, bear a cluster of white, silky, filament-like hairs known as the coma (often referred to by other names such as pappus, "floss", "plume", or "silk"). The follicles ripen and split open, and the seeds, each carried by its coma, are blown by the wind. Some, but not all, milkweeds also reproduce by clonal (or vegetative) reproduction.
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Asclepias
Asclepias is a genus of herbaceous, perennial, flowering plants known as milkweeds, named for their latex, a milky substance containing cardiac glycosides termed cardenolides, exuded where cells are damaged. Most species are toxic to humans and many other species, primarily due to the presence of cardenolides. However, as with many such plants, some species feed upon milkweed leaves or the nectar from their flowers. A noteworthy feeder on milkweeds is the monarch butterfly, which uses and requires certain milkweeds as host plants for its larvae.
The Asclepias genus contains over 200 species distributed broadly across Africa, North America, and South America. It previously belonged to the family Asclepiadaceae, which is now classified as the subfamily Asclepiadoideae of the dogbane family, Apocynaceae.
In the first century AD, Dioscorides described a plant called ἀσκληπιάς (asklepias) in Greek and Pliny the Elder described a plant called asclepias in Latin. Though usually explained as being derived from the name of the Greek medical god Asklepios, neither author mentioned the god in connection with this plant. Their plant has since been commonly identified as the plant now known as Vincetoxicum hirundinaria. Some botanists have disagreed as, though the old descriptions are short and vague, Vincetoxicum hirundinaria does not have leaves like ivy.
Linnaeus first described the genus Asclepias in his Genera Plantarum of 1737 without specifying any particular plants in the genus. Linnaeus published his Flora Suecica in 1745 and Materia Medica in 1749. The only Asclepias he mentioned in either book was the one he would later name as Asclepias vincetoxicum. Both books mentioned that in pharmacy this plant was called Hirundinariæ. In none of his works does Linnaeus give an origin for the name Asclepias, presumably because it was being used for these plants by some herbalists and botanists for centuries before. These publications were before his 1753 innovation of names that were simply two words - genus and species - rather than a much longer description.
When Linnaeus described the 18 species that he assigned to the genus Asclepias in his Species Plantarum of 1753, he included Asclepias vincetoxicum. That plant is now called Vincetoxicum hirundinaria. Of those 18, 7 have been assigned to other genera in the years since: Xysmalobium undulatum, Calotropis gigantea, Gymnema lactiferum, Vincetoxicum hirundinaria, Vincetoxicum nigrum, Gomphocarpus fruticosus and Cynanchum thesioides. The species that remain in Asclepias from those Linnaeus described are: A. syriaca, A. amoena and A. purpurascens both now regarded as A. purpurascens, A. variegata, A. nivea, A. incarnata, A. curassavica, A. decumbens and A. tuberosa both now regarded as A. tuberosa, A. verticillata and A. rubra.
Members of the genus produce some of the most complex flowers in the plant kingdom, comparable to orchids in complexity. Five petals reflex backwards revealing a gynostegium surrounded by a five-membrane corona. The corona is composed of a five-paired hood-and-horn structure with the hood acting as a sheath for the inner horn. Glands holding pollinia are found between the hoods. The size, shape and color of the horns and hoods are often important identifying characteristics for species in the genus Asclepias.
Pollination in this genus is accomplished in an unusual manner. Pollen is grouped into complex structures called pollinia (or "pollen sacs"), rather than being individual grains or tetrads, as is typical for most plants. The feet or mouthparts of flower-visiting insects, such as bees, wasps, and butterflies, slip into one of the five slits in each flower formed by adjacent anthers. The bases of the pollinia then mechanically attach to the insect, so that a pair of pollen sacs can be pulled free when the pollinator flies off, assuming the insect is large enough to produce the necessary pulling force (if not, the insect may become trapped and die). Pollination is effected by the reverse procedure, in which one of the pollinia becomes trapped within the anther slit. Large-bodied hymenopterans (bees, wasps) are the most common and best pollinators, accounting for over 50% of all Asclepias pollination, whereas monarch butterflies are poor pollinators of milkweed.
Asclepias species produce their seeds in pods termed follicles. The seeds, which are arranged in overlapping rows, bear a cluster of white, silky, filament-like hairs known as the coma (often referred to by other names such as pappus, "floss", "plume", or "silk"). The follicles ripen and split open, and the seeds, each carried by its coma, are blown by the wind. Some, but not all, milkweeds also reproduce by clonal (or vegetative) reproduction.
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