Malvales

Malvales, medium-sized angiosperm order, known as the Hibiscus or mallow order, mostly of woody plants, consisting of 10 families, 338 genera, and about 6,000 species. The plants grow in various habitats throughout much of the world, and a number of members are important commercially.

The order is part of the malvids group of core eudicots, which also includes Brassicales, Huerteales, and Sapindales. In earlier botanical systems, some families now placed elsewhere were often associated with Malvales. For example, Elaeocarpaceae was traditionally included in or near Malvales, but is been placed in Oxalidales under modern molecular-based systems. As circumscribed in the Angiosperm Phylogeny Group IV system, Malvales comprised a well supported group of related families. The 10 recognized families in Malvales are typically treated in four groups: Malvaceae, Cistaceae, and Muntingiaceae (with its sister family of Cytinaceae); Bixaceae; Neuradaceae, Thymelaeaceae, and Sphaerosepalaceae; and Sarcolaenaceae and Dipterocarpaceae.

Common characteristics

Many members of Malvales can be recognized easily by their leaves, which often have palmate venation and a rather swollen petiole apex. There are usually bands of fibers in the phloem (the tissue outside the wood), which make their bark stringy or fibrous and tough. Distinctive cyclopropenoid fatty acids are common in members of Malvales, although whether they occur in all families of the order is unknown; mucilage cells are common, as are stipules.

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The flowers often have numerous stamens; there is no nectary disc; and there are usually only a few ovules in each compartment of the ovary. The seed anatomy is very distinctive, with a layer of very much thickened and palisade cells in the inner part of the seed coat. The sepals often meet edge on (valvate) rather than overlapping. The petals, on the other hand, tend to overlap regularly (contorted). The capsule is spiny, and the seeds or the inside of the capsule is conspicuously hairy. The hairs are either in groups, scalelike, or stellate. Looking at the development of the flower, first can be seen just 5 or 10 swellings, or primordia, and later individual primordia of the numerous stamens develop on these swellings. The first stamens to mature are in the center of the flower near the ovary. This is unlike the situation in basal angiosperms, such as members of Magnoliales, which also have many stamens but always develop quite separately, and the first stamens to mature are near the petals.

Malvaceae, Cistaceae, and Muntingiaceae

Malvaceae consists of 243 genera and at least 4,225 species, which are largely tropical but include some plants from temperate regions. The largest or most important genera in the family include Hibiscus (550 species), Dombeya (225 species), Sida (200 species), Pavonia (150 species), Sterculia (150 species), Malva (56 species), and Gossypium (40 species, including cotton plants). Malvaceae now includes several formerly segregate families, mostly woody, with members such as Theobroma (cocoa, or chocolate), Ceiba (kapok), and Ochroma (balsa). Members of the family are usually readily recognizable even when sterile by their combination of fibrous bark, alternate stipulate leaves with toothed margins (the stipules are not sheathing), more or less palmate venation, and multicellular hairs that are stellate to scaly. An epicalyx, the outer series of what appears to be a double calyx, is common and is formed from the bracts or floral leaves of a reduced inflorescence. The sepals meet each other at the edges and are often fused at the base, and closely packed hairs at the base inside secrete nectar. When the nectary is on the inside of the calyx, it probably ensures that the corolla is fused at most only basally. Pollinators—birds and long-tongued insects are common—reach the nectar held in the calyx cup by probing between the bases of the corolla. The edges of the petals overlap regularly. The stamens are often numerous and variously fused or in bundles. The inside wall of the fruit and the surface of the seed are often hairy.

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Cistaceae, or the rock rose family, contains 8 genera and 175 species, which are commonly found in temperate or warm temperate areas, especially the Mediterranean region. Among the major genera in the family, Helianthemum (80–110 species) grows from Europe and North Africa to Central Asia and in the Americas; Cistus (18 species) grows around the Mediterranean and on the Canary Islands; and Lechea (17 species) is native to the New World. Members of Cistaceae are aromatic shrubs, often with opposite leaves that are more or less joined by their broad bases. The inner three sepals are much larger than the two outer sepals. The free petals are crumpled when in bud, and there are numerous stamens. The seed coat is often gelatinous, the endosperm starchy, and the embryo more or less strongly curved, which is an odd combination of characters. Many shrubby species of Cistus have been introduced into North American gardens. C. ladanifer and other species yield pleasantly scented resins that are used in soaps and as scented additives to other products. Helianthemum species, which are all commonly called sun rose, are also occasionally planted by horticulturists.

Muntingiaceae is a small family of three genera and three species from the Neotropics. The family’s leaves are two-ranked, often unequal at the base, and with structures that look like stipules but are in fact the first leaves of the axillary shoot. The flowers are in small groups just above the leaf axil; the sepals meet edge on; the petals are shortly narrowed at the base and are crumpled in bud; and the stamens are numerous. The ovary is very variable, and the fruit is a berry. The Neotropical Muntingia calabura (calabura, jam tree, or Jamaican cherry) is fast-growing and can be used as a firewood crop.

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