Conium


Conium is a genus of flowering plants in the family Apiaceae., Plants of the World Online accepts six species.
All species of the genus are poisonous to humans. C. maculatum, also known as hemlock, is infamous for being highly poisonous. Hemlock is native to temperate regions of Europe, North Africa and Western Asia. The species C. chaerophylloides, C. fontanum, and C. sphaerocarpum are all native to southern Africa.

Description

Plants of the genus Conium are eudicots, flowering plants distinguished by their two cotyledons and tricolpate pollen. They are typically biennial, forming basal rosettes in the first year of growth, and sprouting a rigid, hollow flower stalk in the second. Germination occurs between spring and autumn. Occasionally, plants which germinate in early spring are annual instead of biennial. These plants grow best in wet, poorly drained areas with nutrient rich soil. They grow well in nitrogen rich soil, and are able to tolerate high levels of heavy metals, such as arsenic, cadmium, and lead. Conium plants are diploid, with a chromosome number of 2n = 22. They grow from one to three meters tall, varying between species. They produce hermaphroditic flowers, which are typically insect pollinated or self-fertilized.

Stems

Conium plants are herbaceous, growing non-woody, hollow, and hairless stems. Generally, the stem is striated and light green; however, the coloring varies by species and variety. Some express purple blotching throughout, some have purple blotching localized near the base, and others have no markings at all.

Leaves

The alternate leaves are pinnately compound and finely divided, with the exact pinnation patterns varying between species and leaf type. For example, stem leaves of C. maculatum plants are typically 2-4 pinnate, but basal leaves are 1-3 pinnate. The leaves and petioles are light green. Similar to the stem coloring, some species and varieties express purple blotching on the leaves and petioles.

Flowers

Terminal inflorescences are made up of many small flowers, colored white, yellow, or green depending on the species. The flowers are grouped together in umbrella shaped umbels. Plants in the genus Conium have compound umbels, with multiple dome shaped clusters branching out from a central point. Flowers are around 2 mm across, and the umbrella shaped umbels range from 1–8 cm in diameter.

Fruits

The two seeded fruit is one of the most significant distinguishing factors between species. They are relatively round, slightly flattened, and dark green or brown. Depending on the species, they range from 2.5 to 4 mm long. Some species have distinctive light brown ribs, some have ribs that are noticeably crenulate, and others show relatively unpronounced rib patterns.

Seeds

Each plant typically produces between 1,700 and 39,000 seeds, and 40-85% will germinate in suitable conditions. However, seeds will remain viable for 3 to 6 years after dispersal. Producing a large number of seeds, and growing in a variety of locations, these plants can be considered invasive in some cases.

Chemical composition

Poisonous alkaloid compounds are present in all tissues of Conium plants. As a flower develops into a fruit and matures, the alkaloids present transform from γ-coniceine, to coniine, and finally to N-methylconiine. When ingested, these compounds interrupt the central nervous system, paralyzing respiratory muscles, and finally resulting in death through respiratory failure.

Species differentiation from other plants

Members of the genus Conium can be easily confused with edible plants of the family Apiaceae. Conium plants have leaves similar to parsley and roots similar to parsnip. However, a few characteristics can be used to distinguish the poisonous Conium plants from other harmless Apiaceae plants. For example, Conium leaves and stems release a foul odor when crushed. Additionally, the species C. maculatum stands out with its purple blotching pattern.

Taxonomy

The genus name "Conium" references the Greek word koneios for 'spin' or 'whirl', alluding to the dizzying effects of the plant's poison after ingestion. In the vernacular, "hemlock" most commonly refers to the species C. maculatum, but it also refers to Cicuta species, where both species are similar in physical features, and both are highly toxic.
The genus Conium was erected by Carl Linnaeus in 1753. Several botanists, such as J.F.M. Cannon, G.H. Leute, and J.H. Ross, have historically made the argument that southern African species of Conium has few significant distinctions. Some even claim that the genus has no independent species at all. They argue that the populations in southern Africa "may be the result of the chance introduction of a few individuals which represented genetically a very limited range of the total variability of the species." Some believe that each species of Conium is synonymous to C. maculatum. Others believe that there are two to three distinct southern African species of Conium.

History

Conium was known to ancient Greeks, who used it for its narcotic properties and in capital punishment. It was the method used to execute Theramenes and the means by which Socrates took his own life.
Conium maculatum, also known as poison hemlock, was first described by Carl Linnaeus in his 1753 publication, Species Plantarum. It was the first described species within the genus. "Maculatum" means spotted, referencing the purple blotches characteristic of this species.
Conium chaerophylloides was described by Danish plant collector Christian Friedrich Ecklon and German botanist Karl Ludwig Philipp Zeyher. In 1828, they came together in South Africa, forming a partnership. For the next decade, they built their collection, describing almost 2000 genera and species.
Both C. fontanum and C. sphaerocarpum were described by South African botanist Olive Mary Hilliard and English botanist Brian Laurence Burtt. Together, they published their classifications in the South African Journal of Botany in 1985.

Species

, Plants of the World Online accepts six species:
  • Conium chaerophylloides Eckl. & Zeyh.
  • Conium divaricatum Boiss. & Orph.
  • Conium fontanum Hilliard & B.L.Burtt
  • Conium hilliburttorum Magee & V.R.Clark
  • Conium maculatum L.
  • Conium sphaerocarpum Hilliard & B.L.Burtt

    ''Conium chaerophylloides''

Conium chaerophylloides, growing two to three meters tall, can be distinguished by its unique yellow-green flowers. Groups of many of these small flowers make up obconical umbels. The outer rays form an angle with the point of attachment, giving the umbel a cone-like shape, which fans out from the base of the inflorescence. They produce round, 4 mm long, dark brown or green fruit, with deep, lightly colored ridges.

''Conium fontanum''

Conium fontanum expresses a similar obconical umbel arrangement, but produces slightly larger fruit, and develops the white flowers characteristic of the other species in the Conium genus. The fruit is usually larger than 4 mm long, with ridges of the same dark green or brown color as the rest of the ovary. These plants grow one to three meters tall, depending on the variety.

''Conium hilliburttorum''

Conium hilliburttorum has not always been recognized as a distinct species within the genus. Like C. sphaerocarpum, these plants have white flowers grouped in hemispherical umbels and small fruit. However, these researchers argue that the fruit and ovary anatomy significantly distinguish them from C. sphaerocarpum. They develop similar sized fruit; however, C. hilliburttorum fruit displays prominent ribs, whereas C. sphaerocarpum fruit is relatively smooth. C. hilliburttorum ovaries are covered in tubercules, or small lumpy outgrowths, like C. chaerophylloides fruit. However, C. chaerophylloides plants have yellow or green flowers, and they grow much larger fruit and are much taller.

''Conium maculatum''

Conium maculatum is distinguished by red or purple blotching along the hairless green stem. Some varieties of the southern African species express a moderate degree of blotching at the base of the stem, but most are simply green. It grows white flowers in umbrella-shaped clusters.

''Conium sphaerocarpum''

Conium sphaerocarpum produces similarly colored, but slightly smaller fruit, less than 3.5 mm long. The ovary is relatively smooth, with almost completely flat, inconspicuous ridges. Additionally, the small white flowers are arranged in hemispherical umbels. Each umbel's outer rays protrude horizontally, forming a semi-sphere shape, with the flat edge at the base of the umbel, and the dome pointing away from the point of attachment. This species tends to grow shorter than others within the genus.

Evolution

Conium is a genus within the family Apiaceae. The family Apiaceae originates from Australasia in the Late Cretaceous period. Conium is specifically part of the Apioideae subfamily which split off from the other subfamilies of Apiaceae between 45.9 and 71.2 million years ago in southern Africa.

Toxin development

The evolution of toxicity within Conium plants and within Apiaceae plants has several theories. Apiaceae plants contain secondary compounds which are compounds that are often limited to just a family, genus, or a species of plant that varies among those smaller groups and is unlikely to be essential to the day-to-day physiological needs of the plant. These secondary compounds are used to mediate the plants interactions with other organisms, such as the prevention of herbivory. Gottfried S. Fraenkel in 1959 called the distribution of secondary compounds within Apiaceae and other families reciprocal adaptive evolution. Ehrlich and Raven in 1964 called Fraenkel's process coevolution. They claimed that secondary compounds diversify by emerging as a novel compound that protects a plant from herbivory. Due to this protection from herbivory and mutation or recombination in an insect, the plant can become susceptible to herbivory once more if an insect evolves the means to detoxify or tolerate the toxin created by the secondary compound. With this new trait, the insect has a new adaptive zone with which it can diversify and the plant has a certain set of adapted herbivores. This is seen with Conium, where C. maculatum has an adaptive herbivore, Agonopterix alstromeriana.