Gentianales (Coffees, Dogbanes, Gentians and Milkweeds)

Abstract

The angiosperm order Gentianales includes the Apocynaceae (dogbanes and milkweeds), Gelsemiaceae, Gentianaceae (gentians), Loganiaceae (strychnine plants) and Rubiaceae (coffee and madder plants). They are found worldwide in most climates, but are most abundant in the tropics.

Keywords:

botany;
systematics;
asteridae;
classification;
evolution

Full text

Gentianales (Coffees,
Dogbanes, Gentians and
Milkweeds)
Lena Struwe,
Rutgers University, New Jersey, USA
The angiosperm order Gentianales includes the Apocynaceae (dogbanes and milkweeds),
Gelsemiaceae, Gentianaceae (gentians), Loganiaceae (strychnine plants) and Rubiaceae
(coffee and madder plants). They are found worldwide in most climates, but are most
abundant in the tropics.
Introduction
The angiosperm order Gentianales (subclass Asteridae)
comprises five plant families: Apocynaceae (including
Asclepiadaceae), Gelsemiaceae, Gentianaceae, Logania-
ceae and Rubiaceae (
Table 1
). The families are distributed
worldwide in most climates, but are most abundant in the
tropics. They can be large trees, shrubs, herbs or small
saprophytes. They are characterized by opposite, simple
leaves usually with interpetiolar structures (stipules, lines
or ridges,
Figure 1
). Colleters are often present at the base of
the leaves or inside the calyx (
Figure 2
). They have showy
flowers usually with 4–5 calyx and corolla lobes, and 4–5
stamens attached inside the corolla (
Figure 3
). Some typical
phytochemicals are caffeine, toxic indole alkaloids and
cardiac glycosides (e.g. strychnine, vincristine), and bitter
tasting secoiridoids.
Families Included and Morphology
Rubiaceae (coffees, gardenias and madder plants) is the
largest family in the Gentianales (
Table 1
), and the plants
have distinct interpetiolar stipules and inferior ovaries
(
Figures 1
,
3b
). This family contains mostly tropical trees
and shrubs, but also herbs. Rubiaceae is the fourth largest
angiosperm family.
Apocynaceae (dogbanes, frangipani and milkweeds)
now includes the former family Asclepiadaceae (the
milkweeds, a younger group inside the dogbanes). They
have latex (milky sap), often a tuft of hairs on the seeds
(
Figure 4
), and sometimes pollen fused in tetrads or pollinia
(
Figure 5b
). The younger evolutionary lineages in the group
have intricately formed flowers with fused corollas,
stamens and/or styles with appendages (
Figure 5a
).
Gentianaceae (centaury and gentian) is a medium-sized
family (
Table 1
) with several large herbaceous genera in the
temperate regions. Gentians have contorted (twisted,
overlapping) corolla lobes in buds (
Figure 6
), and have
unique types of the chemicals secoiridoids and xanthones.
Loganiaceae is a smaller family (
Table 1
). They have
imbricate (unevenly overlapping) or valvate (edge to edge)
corolla lobes in buds, and sometimes produce indole
alkaloids. All genera are mainly tropical. Butterfly bush
(Buddleja) had been included in this family, but is now
thought to be most closely related to the snapdragon family
(Scrophulariaceae).
Gelsemiaceae (Carolina jessamine) is the smallest family
(
Table 1
). It has heterostylous flowers and styles that are
branched twice into four stigmas. Only two genera belong
to this family, Gelsemium (North America and southeast
Asia) and Mostuea (Africa and northeastern South
America).
Economically and Horticulturally
Important Species
Many species of this order are poisonous, attractive, and
have many native uses. By far, the most financially
important species is coffee (Coffea, Rubiaceae). Another
important species is Cinchona, the source of quinine, which
is used to treat malaria. The indole alkaloids of Strychnos
and Catharanthus (rosy periwinkle) are used as rat poison
(strychnine) and against leukaemia (vinblastine, vincris-
Article Contents
Introductory article
.
Introduction
.
Families Included and Morphology
.
Economically and Horticulturally Important Species
.
Ecology
.
Phylogeny
.
Biogeography
Table 1 Families of Gentianaceae; number of genera and
species
Family Genera Species
Apocynaceae c. 450 c. 4600
Gelsemiaceae 2 11
Gentianaceae 87 c. 1650
Loganiaceae 10 c. 400
Rubiaceae c. 650 c. 13 000
Total:c. 1200 c. 19 700
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tine), respectively. Gentiana and relatives are used in
traditional medicine worldwide in digestives, inflammation
and snake-bite remedies, and as antifungal agents.
Many Gentianales species are cultivated for their showy
flowers. Some of the most popular are the Rubiaceae and
Apocynaceae genera Allamanda, frangipani (Plumeria),
Gardenia, rosy periwinkle (Catharanthus), and wax flower
(Hoya). Some Gentianaceae are also commonly cultivated
(e.g. Eustoma,Gentiana) and one Gelsemiaceae, Carolina
jessamine (Gelsemium), is grown in North America.
Ecology
Several specialized ecological habits can be found in
Gentianales, e.g. epiphytes, saprophytes without chlor-
ophyll, and co-dependence of pollinators or symbionts and
IS
Figure 1 Stem with opposite leaves and interpetiolar stipule (IS).
Figure 2 Colleter, multicellular gland, in cross-section, showing central
glandular port and multicellular surface.
(a) (b) (c)
Figure 3 Flowers of Gentianales. (a) Symbolanthus (Gentianaceae), with
basally fused calyx and trumpet-shaped fused corolla. (b) Cross-section of
typical Rubiaceae flower showing inferior ovary, calyx, corolla, and stamens
inserted into corolla tube. (c) Cross-section of typical Gentianaceae and
Loganiaceae flower, showing superior ovary, otherwise the same as (b).
Figure 4 Seed from Asclepias (Apocynaceae), showing tuft of hairs on one
end.
(a) (b)
Figure 5 Flower (a) of Asclepias (Apocynaceae), showing intricately
formed appendages on corolla, fused staminal parts around style and (b)
magnified pollinia with ‘handles’ (called translator).
Gentianales (Coffees, Dogbanes, Gentians and Milkweeds)
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plants. Saprophytes have evolved several times in this
order and often have strongly reduced seeds and highly
specialized embryological traits. Gelsemiaceae and Gen-
tianaceae live in symbiosis with fungi (mycorrhiza).
Symbioses with ants are found in, for example Dischidia
(living in leaves, Apocynaceae), and Hydnophytum and
Myrmecodia (living in stems, Rubiaceae). The poisonous
milkweed (Asclepias,
Figure 5a
) is the host plant for the
larvae of monarch butterflies, which accumulate the plant
cardiac glycosides in their bodies as a protection against
predators. Many flowers exhibit morphological traits
specialized for a particular pollinator, such as humming-
birds, bats, hawkmoths or flies. Animal-dispersed fleshy
fruits (berries, etc.) are common and have evolved
independently many times from dry fruits.
Phylogeny
The phylogeny of Gentianales has been well studied using
both morphological and DNA sequence data. Rubiaceae
appears to be the oldest and closest relative to the four
other families (
Figure 7
). Next up in the phylogenetic family
tree is Gentianaceae (including tribe Potalieae formerly in
Loganiaceae). Loganiaceae is then most closely related to a
group consisting of Apocynaceae and Gelsemiaceae.
Apocynaceae and Gelsemiaceae (which was formerly a
subgroup in Loganiaceae) are most closely related to each
other. All five families are monophyletic, with Loganiaceae
as the most weakly supported group.
Biogeography
The biogeographic history of the Gentianales is very
complex and still poorly understood. The oldest groups in
Gentianaceae and Loganiaceae show geographical dis-
tributions that indicate an evolutionary origin during or
before the split-up of the Gondwana supercontinent (an
old, tropical origin for all families). This might suggest that
the evolutionary lineages of the five different families and
some of their major tribes could have been present c.90
million years ago (Ma). Some younger groups show
relationships that suggest a northern subtropical or
temperate distribution that became interrupted and
restricted to smaller areas during the cooling of the Earth
around 38 Ma (the break-up of the Boreotropics vegeta-
tion belt). After the formation of the Panamanian land
bridge about 2–3 Ma, several genera dispersed from North
America into South America. Dispersal from Asia to
North America via the Bering land bridge is also known.
Further Reading
Backlund M, Oxelman B and Bremer B (2000) Phylogenetic relation-
ships within the Gentianales based on ndhF and rbcL sequences, with
particular reference to the Loganiaceae. American Journal of Botany
87: 1029–1043.
Endress ME, Sennblad B, Nilsson S et al. (1996) A phylogenetic analysis
of Apocynaceae s. str. and some related taxa in Gentianales: a
multidisciplinary approach. Opera Botanica Belgica 7: 59–102.
Leeuwenberg AJM (ed.) (1980) Fam. Loganiaceae. Die natu
¨rlichen
Pflanzenfamilien. vol. 28b(1). Berlin: Duncker and Humblot. [In
English.]
Robbrecht E (1988) Tropical woody Rubiaceae. Opera Botanica Belgica
1: 1–271.
Struwe L and Albert VA (eds) (in press) Gentianaceae –Systematics and
Natural History. Cambridge: Cambridge University Press.
Figure 6 Flower bud with contort corolla bud in Gentianaceae. Note
calyx fused at the base and two leaves at the base of the flower.
Apocynaceae
Gelsemiaceae
Loganiaceae
Gentianaceae
Rubiaceae
Inferior
ovaries,
distinct
stipules
Latex
ducts
Indole alkaloids,
interpetiolar
stipules or lines
Xanthones,
no alkaloids
Style
branched
twice,
heterostyly
Figure 7 Synopsis of phylogenetic relationships in Gentianales.
Characteristics for families and groups of families are noted.
Gentianales (Coffees, Dogbanes, Gentians and Milkweeds)
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