Myrtales

▪ plant order

Introduction

      the myrtle order of flowering plants, composed of 14 families, 380 genera, and about 11,000 species distributed throughout the tropics and warmer regions of the world. The majority of these species belong to just two families, Melastomataceae and Myrtaceae. Myrtales includes many trees (notably Eucalyptus), shrubs such as the classic myrtle, several food and spice genera, and many ornamental plants. Myrtales is placed in the basal Rosid group of the core eudicots in the Angiosperm Phylogeny Group II (APG II) botanical classification system (see angiosperm).

Distribution and abundance

      The vast majority of species in Myrtales are shrubs and trees of the tropics and subtropics. Some of the most important sources of pulp and timber in these areas, such as Eucalyptus, belong to the order. Myrtales has few representatives in temperate regions and almost none in cold zones; however, some species occur in Tasmania, and others reach the timberline in southern Australia.

  Myrtaceae, with 131 genera and about 4,600 species, is particularly abundant in Australia and tropical America; some 75 genera and 1,500 species are found in Australia, and about 2,400 species occur in tropical America. Dry-fruited species predominate in Australia, whereas berry-fruited members are found in tropical America. The genus Eucalyptus (including Corymbia), very much an Australian specialty, contains close to 700 species and forms large forests from the semiarid to the wet coastal zones and up to the tree line. Another large genus, Eugenia (including Hexaclamys), has more than 1,100 species. Myrtus grows in the warmer regions of both hemispheres, with M. communis (myrtle) being an important element in Mediterranean vegetation. The monogeneric families Heteropyxidaceae, restricted to southeast Africa, and Psiloxylaceae, endemic to the Mascarene Islands in the Indian Ocean, are placed close to or within Myrtaceae. In addition, the disjunct family Vochysiaceae, with 7 genera and 190 species in the Neotropics and 1 genus of 3 species restricted to tropical West Africa, is placed near Myrtaceae.

      Melastomataceae contains more than 4,500 species in 182 genera. Its members are found along the entire humid tropical belt but are most diverse in the New World, where two-thirds of the species are found. Its largest genus and one of the largest in the flowering plants in general is Miconia, with more than 1,000 species. Most members of the family are shrubs or small trees, but there are some large trees as well as herbaceous perennials and annuals (plants that complete an entire life cycle in one growing season), root climbers, and true epiphytes (epiphyte) (nonparasitic plants that live on other plants).

 The mostly herbaceous Onagraceae, or the evening primrose family, has 656 species in 22 genera widely distributed in nature, although it is chiefly found in the temperate zones of the Americas, particularly in the western regions. The largest genus of the family, Epilobium (willow-herbs), has approximately 165 species and is distributed in temperate zones throughout the world. Fuchsia, with mostly woody members, has 105 species and is mainly distributed throughout the region of the Andes Mountains in South America, with some species in New Zealand and southeastern Brazil. The evening primrose family also comprises several marsh plants and aquatics, such as the genus Ludwigia.

 Lythraceae, or the loosestrife family, containing about 620 species in 31 genera of trees, small shrubs, and perennial herbs, occurs primarily in warmer regions of both the Old World and the New World and is especially diverse in South America and Africa. It now includes the former family Punicaceae, consisting of two species of Punica ( pomegranate); the former family Sonneratiaceae, comprising seven or eight species of two Old World mangrove and tropical rainforest genera, Sonneratia and Duabanga; and the former family Trapaceae, with the single genus Trapa ( water chestnut), with two species of aquatic herbs found from central and southern Europe to eastern Asia and from tropical to subtropical Africa. It has become naturalized in North America and Australia. The largest genus in the family, Cuphea, has approximately 250 species in the American tropics. Lythrum salicaria (purple loosestrife) is originally from the Old World, but its range has extended from Europe and Asia into North America and southeastern Australia.

 Combretaceae, or the white mangrove (or Indian almond) family, with about 500 species in 14 genera of mostly trees and shrubs, is especially important along tropical seacoasts, in African savannas, and in Asiatic monsoon forests. It comprises mangrove species of muddy shores or estuaries; examples include Laguncularia (white mangrove) and Lumnitzera (eastern mangrove), as well as genera of large trees such as Terminalia (190 species) and Combretum (255 species).

      Memecylaceae includes 435 species in 6 genera. Its main centre of development is in tropical Asia, with a second centre in the Amazon basin. Most of its members are trees of lowland rainforests.

      Crypteroniaceae, with about 10 species of trees in 3 genera, is found entirely in Southeast Asia. Alzateaceae consists of one or two species, particularly a scrambling shrub or treelet that occurs from Bolivia, throughout the Andes, to Costa Rica. Three small families related to Alzateaceae and Crypteronianceae are restricted to Africa: Rhynchocalycaceae; Oliniaceae, found in eastern and southern Africa and on the island of St. Helena, with five species of the single genus Olinia; and Penaeaceae, consisting of Penae and 6 other small genera with a total of 23 species of low shrubby habit adapted to life in the dry parts of southwestern and southern Africa.

Characteristic morphological features

      The wood of all Myrtales is characterized by the presence of phloem tissue on each side of the xylem vessels (included, intraxylary phloem), in contrast with the presence of only one phloem layer in most angiosperms (angiosperm). In Myrtales the pits of vessels have a sievelike appearance because of minute outgrowths from their borders, which arch over the pit cavity. Bordered pits with such processes are called vestured pits. This combination of wood anatomical characteristics is otherwise very rare in angiosperms and is used to help define the order.

      An unusual feature seen in most species of Eucalyptus and some other members of Myrtaceae is the presence of lignotubers. These organs are large, woody, rounded outgrowths, up to several centimetres in diameter, surrounding the base of the young tree trunk. The lignotuber consists of a mass of vegetative buds and associated vascular tissue and contains substantial food reserves. If the top of a seedling, which has developed a lignotuber, is destroyed by fire, drought, or grazing, growth is vigorously renewed by the development of new shoots from the lignotuber. It is evident that this organ is of considerable value in an environment where fire and drought are frequent.

 The root systems of mangroves (mangrove) that grow in tidal mudflats, such as Sonneratia (family Lythraceae) and Laguncularia (family Combretaceae), are characterized by the presence of “breathing roots” (pneumatophores), portions of the root that grow upward until they project some centimetres above the low-tide level. They have small openings (lenticels) in their bark so that air can reach the rest of the plant's root system. Another feature of most mangroves is aerial prop roots, which form a tangled jungle, even after the main roots and stem bases of the trees have decayed. In the Onagraceae family, a similar adaptation to an oxygen-deficient environment is seen in Ludwigia, a genus of plants found in wet areas. When plants grow in water, air spaces develop in the tissues of the roots and submerged stems, and in L. repens special spongy-tissued respiratory roots grow to the surface of the water. Under ordinary dry-land conditions, however, the same plants do not exhibit these features. Loose internal cell tissue that contains air spaces also occurs in the submerged portions of the stems and roots of marsh-inhabiting species of the family Melastomataceae.

      Most members of the order bear opposite, simple, and entire leaves (leaf), but there is considerable variation in leaf arrangement between families and within species or even individuals. For example, within a single genus, Quisqualis (family Combretaceae), alternate leaves are borne on the stem, and opposite leaves are borne on the flowering shoots. In Eucalyptus, young branches have opposite leaves, whereas the leaf arrangement on older branches is alternate.

      Whorled leaves occur in a few genera, and quite often the leaves are disjunct-opposite, or “scattered,” as in many species in Myrtaceae, Onagraceae, Combretaceae, and some Lythraceae. In most of the families in the order, stipules are diminutive, only rarely reaching a length of more than a few millimetres; they are absent in Combretaceae, Onagraceae, Melastomataceae, and Memecylaceae. The upper leaves of Trapa and some Ludwigia species have inflated petioles that act as a float, while the submerged leaves are finely divided and resemble roots. In most Melastomataceae and in some Myrtaceae, there are several main secondary veins that branch out from the base of the blade and many tertiary veins that run in a transverse pattern across the leaf blade. This venation pattern gives the leaves of nearly all members of Melastomataceae a highly characteristic appearance. Leaves of Myrtaceae species are further characterized by large scattered oil-containing cells that are visible as translucent spots when the leaves are held up to the light.

      The inflorescences (inflorescence) found in Myrtales are extremely variable, but the panicle seems especially frequent. In some members of Myrtaceae and Melastomataceae, flowers (flower) and fruits are borne directly on the old wood of the trunk; this is known as a cauliflory. The flowers of most species of Myrtales are bisexual (i.e., they have both male and female parts in the same flower), but those of some species of Myrtales, such as Eucalyptus calophylla, Pimenta dioica, some Sonneratia species, and a very few members of Melastomataceae and Crypteroniaceae, have flowers of only one sex; structures of the opposite sex, though originally present, fail to mature in these cases.

      In Myrtales the floral tube (hypanthium) surrounds the ovary either tightly or loosely or is fused to the ovary walls for varying lengths. The rim of the hypanthium bears calyx lobes (free sepals), petals, and either one or two whorls of stamens or numerous stamens. Flowers in which the flower parts appear to arise at the top of the ovary rather than at its base (epigynous) are considered the most advanced, and perigyny (the flower parts appear to arise at the same level as the ovary) is undoubtedly the ancestral condition in Myrtales. The flowers commonly contain four or five parts (four- or five-merous) and very often have two whorls of stamens or stamens grouped in fascicles (bundles). The calyx lobes are sometimes shed as a cap when the flower blooms; the cap may be woody (Eucalyptus) or herbaceous (several genera of Melastomataceae). Flowers of Vochysiaceae are most unusual in the order, with fertile stamen number reduced to one in some species.

      Myrtales pollen is usually in the two-celled state when mature and liberated from the anthers; i.e., it consists of a large vegetative or pollen tube cell and a smaller generative cell. Only in Combretaceae do some species have three-celled pollen grains (with the generative cell having divided to form the two sperm cells). Members of Onagraceae are very distinct in the order in their pollen characteristics. In most flowering plants, the four haploid cells resulting from the nuclear reduction divisions ( meiosis) in the anthers, and from which the pollen grains (the male gametophytes (gametophyte)) develop, remain joined together for only a very brief period before separating as four individual pollen grains. In Onagraceae, however, they often form tetrads (groups of four joined at their corners), and, in most members of the family, the pollen is furthermore connected by viscid threads that stick to a visiting insect; thus, hundreds of pollen grains are drawn out of the anther at the same time.

      Following fertilization, nuclear endosperm, a tissue with nutritive function for the developing embryo, is formed in the embryo sac. This tissue is depleted during seed development, and the mature seeds are generally without any or with a very thin layer of endosperm. In the water chestnut, however, endosperm formation hardly takes place. Instead, the embryo sac becomes prolonged and invades the surrounding tissues, from which the embryo is then supplied with nutrients. Although normally the embryo in Myrtales species has two cotyledons and stores fatty oils and aleurone, the water chestnut has only a single rudimentary cotyledon, and the embryo stores starch. Starchy embryos also occur in some Myrtaceae and Memecylaceae species.

      In Eucalyptus, only relatively few of the ovules present in each ovary develop into seeds; the majority are sterile. The sterile ovules, called ovulodes, remain small and form chaff. The amount of mature seed to chaff by weight is often as little as 5 percent. polyembryony, the presence of several embryos in each seed, on the other hand, occurs in several families of the order and is very common in myrtle and the genus Eugenia (including the Brazilian and Surinam cherries). In E. paniculata, up to 21 embryos have been found in a seed, and it is unusual for there to be only one.

      Vivipary (viviparity), a feature of mangroves (mangrove) such as Sonneratia and Laguncularia, is seed germination inside the fruit while it is still retained on the tree; following the growth of the seed leaves, the root end of the embryo elongates and passes through the opening in the seed (micropyle) and then through the ovary wall. It may grow a considerable length before the seedling drops into the mud, where it establishes itself at once.

Economic and ecological importance

 The most economically important single genus in the order is Eucalyptus, a hardwood plant. The growing demand for wood, pulp, and paper, as well as other reconstituted uses such as hardboard, has made the genus one of the most widely cultivated plants in warm temperate and tropical parts of the world. One reason for this is its fast growth rate. Several species of trees are 30–50 metres (about 100–165 feet) tall; for example, the Australian mountain ash of Victoria and Tasmania, E. regnans, may attain a height of more than 90 metres (almost 300 feet). Ethereal oils with fever-controlling and germicidal properties, gums, and resins are extracted from Eucalyptus leaves and wood. Some species are an important source of nectar and pollen for honeybees. Other members of the order used for timber include Metrosideros, Angophora, and Syzygium. Physocalymma scaberrima (Brazilian tulipwood), Lagerstroemia flos-reginae (pride-of-India), several species of Terminalia, such as T. alata (Indian laurel), T. procera (white bombway), T. superba (afara, or limba), and T. brassii (swamp oak), are used in reforestation programs in swampy tropical lowlands.

      Although the order is not an important source of food, edible fruits (fruit) are produced by some of its members. The most widely known is probably Punica granatum ( pomegranate), which is now cultivated in the warmer regions of the world for its fruit and as an ornamental shrub. In Myrtaceae, Psidium guajava ( guava), P. cattleianum (strawberry guava), Feijoa sellowiana (feijoa, or pineapple guava), and many species of Eugenia and Syzygium, such as Eugenia dombeyi (Brazilian cherry), E. uniflora (Surinam cherry), Syzygium jambos (rose apple), and S. malaccense (Malay apple), are of local value. Their fruits are eaten raw or cooked and are used for making jellies, preserves, and beverages. Small industries have grown around Psidium guajava in many warmer parts of the world, such as Florida, Colombia, and Brazil; guava contains more vitamin C than most Citrus fruits. The boiled fruits of the water chestnut are popular from southern China to Thailand; in northwestern India and Kashmir, flour is prepared from them. Although the fruits of all the berry-fruited members of Melastomataceae are edible, only one species, Bellucia pentamera (also known as B. axinanthera), has been considered a potential fruit crop, and it was introduced for this reason into the Old World by the Dutch. Combretaceae supplies Terminalia catappa (Indian, or country, almond), and in tropical Africa a butterlike substance called chiquito is obtained from the fruits of Combretum butyrosum.

 Because of the essential oils (essential oil) present in secretory cavities in members of Myrtaceae, this family provides valuable spices such as the dried flower buds of Syzygium aromaticum (clove tree), the cloves (clove) of the spice commerce. Clove oil, however, which used to be commercially extracted from young parts of S. aromaticum, is now synthetically produced. Medicinally important oils containing cineole, limonene, or citral are extracted from several species of Eucalyptus, including E. dives (broad-leaved peppermint) and E. radiata (narrow-leaved peppermint). Pimenta is a genus of aromatic trees. Pimenta racemosa, for example, yields bay oil, which is used by the perfume industry, as are the oils of some Eucalyptus species and the powdered unripe fruit of P. dioicais allspice (pimento).

      Lawsonia inermis, of the family Lythraceae and native to northeastern Africa, is the henna of commerce, yielding an orange-red dye that has been used for centuries in the Middle East and East Asia for colouring the hair, fingernails, and soles of the feet. The leaves contain a substance that reacts directly with the keratin of human hair and skin to form the bright pigment. Chemically altered henna is now used as the base for a wide array of hair colorants. Tannins (tannin) for tanning hides are extracted from the bark and foliage of the Australian Eucalyptus astringens and from the fruits (myrobalans) of Terminalia chebula.

 Lagerstroemia indica (crepe myrtle), from family Lythraceae, originating in tropical Asia and Australia, is a popular garden shrub or tree widely cultivated for its beautiful pink, purple, or white flowers arranged into panicles and for its smooth gray bark. Species of Quisqualis and Terminalia catappa (Indian almond), the former a liana and the latter a tree, are also cultivated in the tropics.

 Other horticulturally important plants in Myrtales include species of Eucalyptus, myrtle, Chamaelaucium unicatum (Geraldton waxflower), Callistemon (bottlebrush), Feijoa, and several horticultural hybrids of Leptospermum.

      Eucalyptus has six subgroups that are derived from six different bark types: peppermints, with fibrous bark; stringbarks, with stringlike fibrous bark; boxes, with rough bark; bloodwoods, with rough scaly bark; gums, with smooth bark; and ironbarks, with hard bark.

      Melastomataceae contains Tibouchina organensis (glory bush), with its striking purple to violet flowers and purple anthers, often cultivated outdoors in the southeastern United States and elsewhere in the warm tropics. Some of the more beautiful greenhouse plants of Melastomataceae are Medinilla magnifica, whose purple flowers are arranged in pendulous panicles up to one foot long and subtended by pink bracts 2.5–10 cm (1–4 inches) long, and various species of Bertolonia, Monolena, and Sonerila, which are cultivated for their interesting foliage.

 Herbaceous annuals or perennials of Onagraceae, such as those found in Oenothera (evening primroses), are popular cultivated ornamentals in which the scented flowers open in the evening. Another ornamental in the family is Clarkia amoena (Godetia). The shrubby Fuchsia is a cultivated plant familiar throughout the world for its showy flowers in delicate, usually pendulous inflorescences. The flowers generally are in shades of red and purple, with some parts white. The calyx tube (hypanthium) is bell-shaped to tubular and prolonged beyond the ovary. The four calyx lobes remain free and spreading; in some species the petals are lost.

Natural history

      In general, the flowers of Myrtales are adapted for pollination by animals, because they frequently provide abundant nectar or pollen and are shaped so as to facilitate pollen deposition on the stigma while the animals forage on these foods. A highly specialized mechanism for promoting outcrossing is widespread in Lythraceae, where members of Lythrum, Decodon (swamp loosestrife), and Nesaea have three flower forms on different plants (trimorphism); plants with two flower forms (dimorphic) are known in Rotala (toothcup) and in Lythrum. As such, the style and stamens of a flower differ in length, and the pollen of the different stamen whorls differ in size and chemical characteristics, as do the stigmatic papillae. Only pollinations between anthers and stigmas of the same height (necessarily from different plants) result in the production of seed. The pollen from the different anther levels is carried on different places on the bees and butterflies that visit the flowers for the nectar and thus effect the obligate cross-pollination. Although Lythraceae flowers normally open when mature, the opposite condition, of mature flowers remaining closed (cleistogamy), occurs in Ammannia and is thought to occur in the apetalous species of Rotala, Lythrum, and Nesaea.

      Most species of Combretaceae are pollinated by insects, although some are pollinated by birds. This is also true of the Onagraceae family—e.g., Fuchsia and some evening primroses and Epilobium (willow herbs) species are bird-pollinated. In addition, some species of Epilobium have many small-flowered largely self-pollinated species. The transfer of pollen in Myrtaceae is usually accomplished by insects, chiefly bees, and to a lesser extent by moths and butterflies. Some species of Eucalyptus and Metrosideros are visited for nectar and pollinated by birds, especially honey eaters. Memecylaceae and most Melastomataceae species have nectarless flowers adapted to pollination by pollen-collecting bees. In many members of Melastomataceae, the stamens bear conspicuous, often yellow appendages and may be of two lengths or different colours; it is not clear whether these elaborations have a function in the pollination mechanism beyond that of enhancing the visual attractiveness of the flowers and making the stamens easier for the bees to grasp. Some 80 species in perhaps 11 genera of the family Melastomataceae offer nectar as a reward to pollinators; they are pollinated by hummingbirds, bats, and rodents, as well as by bees and wasps. Flowers of Trapa (family Trapaceae) are predominantly self-pollinated, and those of the other small families of the order are pollinated by birds (some Penaeaceae) or insects (Alzateaceae, Crypteroniaceae, and Rhynchocalycaceae). Sonneratia is pollinated by bats and hawk moths. The pollinating mechanisms of Oliniaceae are not completely known.

      Seed dispersal (dispersion) in the order is by wind, water, or animals, depending on whether the fruits are dry and capsular or fleshy—both types often occur within the same family. Melastomataceae and Myrtaceae berries, particularly those from underlying low-growth vegetation in forests, are important food for birds; their seeds are viable after passing through the gut. Seeds of many members of Lythraceae and Combretaceae are well suited to water dispersal by means of winglike expansions of the seed coat or by buoyant airy tissues on the outer seed coat. Some seeds of Terminalia (family Combretaceae) are able to float for several months in seawater without adverse effects on germination.

      In some members of Melastomataceae, such as Maieta, Tococa, and certain Clidemia, the leaf bases develop saclike outgrowths that serve as shelters for ants (ant), which enter them through two small holes on the lower surface. The ants protect the leaves from herbivores such as caterpillars, which they kill or drive away.

Evolution

      Among fossil flowers and fruits from the Maastrichtian Stage (70.6 to 65.5 million years ago) of the late Cretaceous Period, Myrtales species are especially well represented, and pollen of the Myrtaceae family has been reported from the Santonian Stage (85.8 to 83.5 million years ago). Even aquatic plants in the order, such as Decodon (family Lythraceae), are represented among fossils from the late Paleocene Epoch (65.5 to 55.8 million years ago) of the Tertiary Period. Thus, the families of Myrtales, as they are now defined, were probably all in existence by Tertiary times, and their common ancestors existed in Gondwana before the separation of South America from Africa during the Jurassic Period (199.6 to 145.5 million years ago).

      Molecular information has yet to convincingly identify close relatives of Myrtales within the Rosids, but DNA sequence analyses suggest that the order may be closest to Sapindales, Brassicales, and Malvales in the Rosid II group. Myrtales is composed of two major lineages. The first includes Combretaceae, Lythraceae, and Onagraceae, and the second contains the remaining eight families. Lythraceae and Onagraceae are particularly closely related in the first lineage. Within the second lineage, the large tropical families Myrtaceae and Melastomataceae, with related smaller families, form two separate evolutionary lines. A cluster of five small families, Alzateaceae, Oliniaceae, Penaeaceae, Rhynchocalycaceae, and Crypteroniaceae, forms a closely related assemblage near Melastomataceae and Memecylaceae. Of these five, the three African families—Oliniaceae, Penaeaceae, Rhynchocalycaceae—are closely related. Two other African families, Heteropyxidaceae and Psiloxylaceae, also form a closely related assemblage but near Myrtaceae and Vochysiaceae.

Susanne S. Renner Kenneth J. Sytsma

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