Myrtle Family (Myrtaceae)

Species of the myrtle family provide many valuable products, including timber (e.g. Eucalyptus), essential oils (e.g. allspice), and horticultural plants (e.g. ornamentals such as Verticordia and food plants such as Psidium, guava).

The various species of the Myrtaceae are sources of several valuable essential oils, produced by distillation from leaves. Many of the components of these oils are based on the isoprenoid unit, five carbon atoms linked together in a branched structure with a double bond between two of the carbon atoms involved in the branch point.

Eucalyptus oil includes eucalyptol, with the major component being 1,8-cineole, a modified monoterpenoid, along with hexanol and caproaldehyde. The composition of essential oils varies with the species from which they are distilled, giving each oil a unique character. Eucalyptus risdoni contains tasmanone, while E. grandis contains grandinol. One species of Leptospermum is used to produce a lemon-scented oil, including the compound leptospermone, and Myrtus yields the oil known as Eau d'Anges (literally, water of the angels).

Eucalyptus trees near Alice Springs, Australia. JLM Visuals. Reproduced by permission.

Cloves and allspice also come from the Myrtaceae. Cloves, one of the best known spices from the myrtle family, comes originally from the islands near Southeast Asia. The spice is composed of the whole, dried immature flower buds of the species Szygium aromaticum. Grown in Indonesia, Malaysia, and several African countries, the name for this popular spice comes from the Sanskrit Katukaphelah, which became the Greek karyophyllon and from there the English clove. The buds of this 38-49 ft (12-15 m) tree must be picked with very precise timing and sun-dried to avoid fermentation.

Cloves are a flavoring for mulled cider and ketchup, and for decoration in pomanders. Dried apples or oranges covered with cloves are valued for their pleasant scent. The scent of cloves was considered so pleasant that courtiers of Han Dynasty China chewed cloves to improve their breath before appearing in court. Cloves give their flavor and name to clove cigarettes, or kretek in Indonesia. They are made by mixing cloves with the tobacco.

Distilled clove oil was once used by dentists as an anaesthetic. Cloves were no longer used after some patients had adverse reactions. The principal component of this oil is eugenol, a complex alcohol based on the benzene ring, making it an aromatic chemical.

Allspice, or bay rum, also comes from the Myrtaceae. The spice is extracted from dried, unripe, peasized berries of the species Pimenta dioica, which are reddish brown in color. Unlike cloves, which are native to the Old World, the tall trees which produce these berries grow only in areas near their native climate of Central America and the West Indies. For this reason, the original producers had their business protected. Many other spices were eventually grown in places different than their origins, which hurt the economies of the original producing nations. Most worldwide production comes from the Caribbean—Jamaica, Haiti, Guatemala, and other nearby nations. The name comes from the Spanish pimienta, or black pepper. This confusion of names comes from the similar appearance of the pepper corns and the allspice berries. Pimienta also applies to the hot pepper, of the Solanaceae which also includes the tomato and the potato.

Allspice flavors a number of common foods including several liquors, such as Benedictine, pies, and ketchup. The principal active compound in allspice is eugenol, as in the case of cloves.

Many species from the myrtle family have attractive glossy green leaves and colorful flowers making them popular horticultural plants. Many species are from arid regions, so the drought tolerance provides another valuable trait for horticulture. They are particularly valuable in regions with similar Mediterranean climates, such as southern California, where they remain bright and attractive during an otherwise bleak time of the year. Eucalypts make attractive street trees.

Their resilience has a negative side in the face of drought. The leathery leaves of Myrtaceous plants are rich in highly flammable hydrocarbons and present a fire hazard.

In their natural environments, this abundance of flammable oils acts to promote the survival and success of myrtaceous plants. Forest and brush fires occur normally during the dry times of the year in places where these species of the Myrtaceae live. The flammability of the myrtaceous plants feeds these fires. Species of Eucalyptus regenerate after these fires either from dormant buds or lignotubers, woody subterranean modified stems. Other species which might otherwise compete with Myrtaceous plants for resources such as water and sunlight are destroyed or damaged in the fires while Myrtaceous species, such as eucalypts, survive. Ironically, the ability to burn is actually a competitive advantage for these plants.

The myrtle family provides a number of important timber species, including those of the genera Eucalyptus and Melaleuca. Eucalyptus provided the Australian aborigines the opportunity to make dishes and canoes. The roots of some species were used for food.

Species of Eucalyptus were first collected by expedition botanists traveling with Captain James Cook. The name Eucalyptus was created by botanist, David Nelson, on Cook's third voyage, from the Greek words for "well covered." The name came from complete covering of some parts of the flower bud by the operculum, or dome covering the male and female organs. The operculum sheds when the flower opens. The operculum is a ring of fused, hardened petals, as is found in the non-myrtaceous plant honeysuckle (Lonicera, Caprifoliaceae, soft petals). Species in the genus Eucalyptus, like the rest of the Myrtaceae, are woody perennials ranging in size from shrubs to tall trees.

The genus Eucalyptus is divided into six general types of eucalypts based on the appearance of the bark: the gums with their smooth outer bark resulting from annual shedding of the roughest outermost layer (Eucalyptus apodophyolla); the bloodwoods with tesselated bark, with the dead, outermost layer being composed of plates of short fibered material (E. tesselaris); the boxes, not to be confused with boxwood from the family possess short-fibered bark, which sticks to the outside of the tree after the outermost layer is dead, fading to a pleasant gray (E. moluccana), while the stringy barks also retain the outermost, dead layer of bark but take their name from its long fibered nature (E. reducta) peppermints; the ironbarks (E. melanophloia) are covered with a hard, furrowed bark which remains continuous after the outermost layer is dead. Finally, the minniritchi eucalypts have a partially detached outermost layer of bark which splits longitudinally to reveal the green inner bark (E. orbifolia). The ironbarks are known for the density and lasting quality of their wood. It is a particularly useful wood for heavy building and for railway ties.

Melaleuca, a genus known by the common name paperbark, yields ornamental timber. The various species of the genus include shrubs and trees which grow in swamps and marshes. The flowers resemble bottle brushes, showing the close relationship between this genus and Calliostemon. With the species of the genus Leptospermum, those of the genus Melaleuca share the common name tea-trees. It is from Melaleuca, not Leptospermum, that tea-tree oil is distilled.

Species of the genus Melaleuca were transferred to Florida for development purposes. People wanted to use the strong invasive roots of this tree to turn swampland into firm ground for building. However, in the swamps of Florida, Melaleuca has proven to be an invasive pest that has displaced many native species from the swamps. Melaleuca reproduces so quickly and so thickly that attempts to eliminate it from the swamps by cutting have met with little success. Efforts are being made to remove Melaleuca by introducing an insect which feeds on it. A similar phenomenon has occurred in New Caledonia, where when native trees have burned, solid stands of Melaleuca replaced them.

Besides spices and timber, humans also use species of the Myrtaceae for food. The popular fruit guava is produced by species of several genera from the Myrtaceae. Pineapple guava grows on trees of the genus Feijoa, and strawberry guava comes from Psidium littorale, native to tropical America.

A number of species from the Myrtle family are used horticulturally or decoratively. Attractive flowers are produced by species such a Leptospermum and Calliostemon. Now grown in gardens around the world, species of Calliostemon were originally found growing in wet spots in Australia. Their bright flowers look like long round brushes, thus the common name for this genus, bottle brushes. Some species are unusual in that they are among the few plants in the world pollinated by birds instead of the wind or insects.

The flowers of Leptospermum resemble flattened roses, and the small dark green leaves resemble those of the herb rosemary. Before tea was available in Australia, the leaves of Leptospermum were used as a substitute for tea, which is made from the dried leaves of a species of Camillia. From this early use by European settlers, the genus Leptospermum derives the common name tea-trees. Young branches and dried seed pods of Eucalyptus are popular in floral arrangements. The young branches have the attractive round leaves tightly appressed to the stem and are characteristic of juvenile material from species of the Myrtaceae. The leaves are mottled with wax.