Agavoideae

The agavóideas (scientific name Agavoideae) form a subfamily of monocotyledonous plants distributed more or less throughout the world and especially diverse in Mexico, where Agave has important economic uses (for example, it is used to make tequila and mezcal). Many members of this subfamily are arborescent, such as the Joshua tree (a species of Yucca). The leaves are often large, xeromorphic, fibrous, or rarely succulent, in rosettes at the base or end of the branches. The flowers have 6 tepals and 6 stamens, and the ovary is superior or inferior. The subfamily was recognized by modern classification systems such as the APG III classification system (2009) and the APWeb (2001 onwards), and had already been recognized by the APG II which left the option of excluding it from an Asparagaceae sensu stricto as the family Agavaceae (see Asparagales for a discussion of these clades).

Description

Usually large rosette herbs, or trees, or shrubs, or subshrubs. Rhizomatous, some with stems with abnormal secondary growth. They present raphid-type calcium oxalate crystals and steroidal saponins. simple hairs. They are acaulescent (they lack a stem except the one that gives the flowers and fruits), or they have a rhizome, or bulb, or they are arborescent. In taxa with stems, they are sympodially branched.

Leaves often large, xeromorphic, fibrous or rarely succulent, alternate and spiral, in rosettes at the base or end of the branches, simple, with an entire margin to spiny-serrate, and usually with a sharp spine at the apex, with parallel venation, the vascular bundles associated with thick and strong fibers, sheathing at the base, without stipules.

The inflorescence is a panicle, a cluster, or a spike in some that produce vegetative seedlings.

Flowers usually bisexual, radial to slightly bilateral, often showy, bracteate, hypogynous or epigynous.

The perianth is biseriate (it has two whorls), homochlamydeous (6 equal tepals arranged in two whorls of 3 pieces each), separated to connate, and then tubular to bell-shaped perianth, imbricate, petaloid, not dotted, and usually white to yellows. Hypanthus present in some.

Stamens 6, filaments separate, sometimes adnate to perianth, filaments long and slender to short and thick. Dorsifixed, versatile, longitudinal and introrse dehiscence anthers, tetrasporangiate, ditheca.

Monosulcate pollen.

3 carpels, connate, 3 locules, superior or inferior ovary, with axillary placentation, solitary style, minute stigma, capitate to 3-lobed. Ovules more or less numerous in each locule and in 2 rows per carpel, anatropous, bitegmic.

They present nectaries in the septa of the ovary.

The fruit is a loculicidal or septicidal, or indehiscent capsule, but is sometimes fleshy and berry-like.

The seeds are flattened, the seminal coat with a black crust (with phytomelanins) and the inner layers more or less collapsed.

The karyotype is usually 5 large chromosomes and 25 small chromosomes.

See Verhoek (1998) for additional information on the family.

Ecology

Agave chiapensis in bloom.

Habits of YuccaJoshua's tree.

Widely distributed in warm-temperate to tropical regions of the New World, and especially diverse in Mexico, but introduced to the Old World.

Characteristics of arid and semi-arid habitats. Many times they have CAM-type photosynthesis.

The showy flowers of Yucca and Hesperoyucca are visited by small moths of the genus Tegeticula that have a symbiotic relationship with these plants, the moths females carry pollen and oviposit in the ovaries (the larvae that develop inside eat some seeds). Other genera are pollinated by birds (many Beschornea species) or bats (many Agave species) or bees.

The black seeds are typically dispersed by wind, and the fleshy-fruited species are dispersed by animals.

Phylogeny

Agavoideae is related to Scilloideae (Hyacinthaceae) and Brodiaeoideae (Themidaceae), and the phylogenetic position of some genera is problematic. However, both phenotypic and DNA characters support the monophyly of the subfamily (Bogler and Simpson 1995, 1996, Bogler et al. 2006, Chase et al. 2000, Pires et al. 2004).

Camassia, Hastingsia, and Chlorogalum (usually treated in Hyacinthaceae, as the subfamily Chlorogaloideae) and Hosta (usually placed in Hostaceae) are included here (see Chase et al. 1995a, 2000, Bogler and Simpson 1995, 1996, Bogler et al. 2006, Pfosser and Speta 1999). Hosta is a genus of rhizomatous herbs with broad-bladed leaves with prominent parallel veins, one-sided clusters of lily-like flowers, and black seed capsules. These 4 genera also have a distinctive bimodal karyotype.

Molecular data (Chase et al. 1995a, 2000, Rudall et al. 1997b) also support the inclusion here of Anthericum, Chlorophytum and allies (usually placed in Anthericaceae), a group of rhizomatous herbs with leaves in a basal rosette, leading to a "broad sense" Agavoideae, which is not easy to characterize.

Woody Agaves are usually divided into the Yucca clade (containing for example Yucca, Hesperaloe, Hesperoyucca) with a superior ovary and tiny anthers, and the Agave clade (containing for example Agave, Furcraea, Manfreda, Polianthes), with an inferior ovary and elongated anthers (Dahlgren et al. 1985). Both are monophyletic (Bogler and Simpson 1995, 1996).

The subfamily has often been circumscribed more broadly (Cronquist 1981), including genera that are here included in the Nolinoideae (formerly Ruscaceae, Nolina, Dasylirion, Beaucarnea, Dracaena, y Sansevieria), and Cordyline (in Lomandroideae, formerly Laxmanniaceae). A subfamily so defined is morphologically heterogeneous, unified only by woody habit, and clearly polyphyletic (Dahlgren et al. 1985, Chase et al. 1995a, b, Bogler and Simpson 1995, 1996, Rudall et al. 1997a).

Taxonomy

The subfamily was recognized by APG III (2009). The subfamily had already been recognized by APG II (2003) which left the option of excluding it from an Asparagaceae sensu stricto such as the Agavaceae family.

Pasithea.

The subfamily consists of 25 genera and 637 species (Judd et al. 2007). The most represented genera are Agave (300 species), Chlorophytum (150 species), Anthericum (65 species), Yucca (40 species) and Hosta (40 species).

Genres

Synonymy

These names are synonymous with one of the Agavoideae tribes, according to APWeb (visited January 2009): Anemarrhenaceae, Anthericaceae, Behniaceae, Chlorogalaceae, Funkiaceae, Herreriaceae, Hesperocallidaceae, Hostaceae, Nolinaceae, Yuccaceae.

Economic importance

Many species of Agave, Furcraea, and Yucca are used as a source of fiber, for example the leaves of Agave sisalana .

Agave fourcroydes is the source of henequen.

A few species of Agave are fermented to produce tequila (the fermented and distilled young flowering stems of Agave tequilana) and mezcal.

Both Agave and Yucca are used in the manufacture of oral contraceptives (due to their steroidal saponins).

Some indigenous cultures use plants of this family as a source of fiber, food, drinks, soap and medicines.

Finally, many genera, such as Agave, Hosta, Manfreda, Polianthes, and Yucca, are used as ornamentals.