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Yuccas are usually easy to distinguish from agaves even when out of flower. Like all members of the family, they bear leaves in rosettes. But yucca foliage is only semisucculent or nonsucculent and the leaves are usually straight; many species grow trunks. When in bloom, all are easily recognized by their large, white, fleshy, bell-shaped flowers. Unlike most agaves, most yuccas are polycarpic (blooming more than once).

Range

Like agaves, most yucca species occur in semiarid habitats above the desert. Habitats range from the northern Great Plains through woodlands and the dry tropics of Mexico. One species occurs in the southeastern U.S. and the West Indies. About 10 species occur in the Sonoran Desert region.

Ecology

Yucca pollination ecology is an example of a tight symbiosis called a mutualism. (Symbiosis refers to a close association between two species in that at least one benefits from the association. Mutualism is a symbiotic relationship in which each species depends on the other for survival.) With only one exception, yucca reproduction depends on moths (genera Parategiticula, Tegiticula) which deliberately cross-pollinate the flowers. (Yucca aloifolia of the southeastern U.S. is pollinated by bees.) The blossoms need pollen from a different plant in order to produce seed, and it must be packed into a deep receptacle on the stigma, an event that could not occur by chance visitation. The moth is equally dependent on the yucca. It lays eggs on each pollinated ovary, and the hatched larvae eat some of the developing seeds.

Biologists have only recently determined that almost every species of yucca has its own species of yucca moth; some yuccas have two moth species. Such a tight mutualism has risks for both partners. Emergence of adult moths must coincide with yucca flowering for the reproductive needs of both species to be met. However, the synchronization of moth emergence with flowering is frequently poor and seed set and moth reproduction in such years are low. Furthermore, yucca populations may flower sparsely or not at all in dry years. Yuccas don't have to set seed every year because they flower many times in their long lives. The yucca moths employ a survival strategy analagous to that of desert annual plants. The full-grown larvae exit the ripening yucca fruit and burrow into the ground, where they enter a deep dormancy (diapause). Like the seeds of many annuals, only some of the larvae will metamorphose and emerge as moths in the following flowering season. The rest remain in diapause for two or more years.

Ethnobotany

Yucca flowers and fruits are edible fresh or dried. Chemicals in the roots of some species are used to make soap. (The Spanish name amole is applied to a number of unrelated plant species from which soap is made.) The roots of Mohave yucca (Y. schidigera) were used to provide the foaming agent in root beer, and the stems are still (over-) harvested to produce livestock deodorant.

Behind the formidable appearance of agaves and the plants formerly grouped with them is a wealth of uses. They have long been and still are used extensively by indigenous peoples throughout North and Central America for food, fiber, and medicine. The roasted, sugar-rich agave hearts have been an important food for numerous Native American groups. Juice from the mature plants is consumed both fresh and fermented. Fermented liquid from the cooked heads is distilled into mescal. Tequila, the best-known variety of mescal, is distilled from one species, Agave tequilana. The tequila agave (mezcal azul) is a significant economic crop in southern Mexico; North Americans alone consume more than a million gallons of tequila a year.

Fiber from the leaves of Agave sisalana is the source of sisal rope, and A. fourcroydes yields henequen fiber. Sisal is a major economic product widely cultivated in Africa, Asia, as well as in the New World; it provides 70 percent of the world's hard, long fiber for ropes, rugs, and bags in recent decades. Numerous native American peoples weave baskets from the fibers of yuccas and nolinas.

The complex chemicals in this family have many uses. Compresses for wounds have been made from macerated agave pulp, and juices from leaves and roots were used in tonics. But beware — sap from many agaves can cause severe dermatitis. The juice of the more virulent agaves has been used as fish poison and arrow poison. Agaves and yuccas are used in Mexico to make soap. Yuccas were once used to provide the foam of root beer and are still used in livestock deodorant. More recently steroid drugs have been synthesized from extracts of several species in the family.

Today these plants are appreciated for their beauty and are widely used to add accents to landscape designs and mark property boundaries all over the world. Howard S. Gentry's book Agaves of Continental North America was so popular that it was reprinted in 1992, an unusual event for a botanical monograph.

As originally described by Gentry, Agavaceae consists of 18 genera and a little over 400 species, many of them native to western North America. This family is difficult to define; it has been revised by taxonomists several times in recent years.

Agaves, yuccas, and relatives were once included in the very large lily family Liliaceae. Gentry segregated agaves, yuccas, and other genera into their own family Agavaceae in the 1970s. More recently other botanists have split the old Liliaceae into many more families and removed some genera from Agavaceae, among them Nolina, Dasylirion, Sansevieria, and Dracaena. Whatever their taxonomic status, these are highly useful plants with dramatic forms.