The spiderwort family (Commelinaceae) is a small family of monocotyledonous (with one seed leaf) plants, found primarily in tropical and desert areas of the world. The family contains 38 genera and about 600 species. All members of the family are herbaceous, and are easily recognized by their simple, linear leaves, and large, brittle nodes. Their flowers are borne either on a terminal inflorescence, or flower cluster, known as a cyme, or in leaf axils. Flowers have a single ovary composed of three fused carpels (the future seed) that is placed above the flowers. In cross section, there are three distinct locules. The anthers open through terminal pores, rather than through a lengthwise slit. The fruits are capsules.
In most genera of spiderworts, the flowers are regular and actinomorphic, meaning that they can be bisected along more than one axis to form identical halves. Some spiderworts have irregular flowers. All members typically have three brightly colored petals. Irregular flowers are common in some species in the genus Commelina. Species of Commelina, commonly called dayflowers, may have flowers with three similar petals, as in a typical actinomorphic flower, and with anthers or carpels which curve away from the floral center. Other Commmelinas have flowers in which one of the three petals is reduced in size and unpigmented, or they may have off-center anthers or carpels, and are irregular in shape. One story describes how the Swedish naturalist and taxonomist Linnaeus named the genus Commelina after the three Commeline brothers. Two of these three brothers were famous botanists, but the third was a lawyer. In Linneaus” nomenclature, the two brightly colored petals of Commelina represent the botanically inclined brothers, while the third, insignificant petal represents the lawyer.
The largest number of species in the Spiderwort family belongs to the genus Tradescantia, which includes species native to temperate North America, tropical Mexico, and South America. Temperate North American species of Tradescantia typically inhabit moist lowlands, wet meadows, and wetlands, while many of the tropical species occur on moist mountain slopes or dry uplands. Tradescantia species typically have large, blue, purple, or occasionally white flowers. The best-known species, T. virginianus, was introduced to Europe in 1637, and quickly became a popular garden plant. Today, hybrids derived from species are popular ornamentals sold under the name T. X andersonii.
Other species such as T. occidentalis and T. ohiensis are native to the tallgrass prairieecosystem of the United States. A larger number of species, including T. fluminensis and T. blossfieldiana, are found in the tropics of Mexico and Central and South America.
All Tradescantia species have a similar growth form, and many are used as garden ornamentals. However, a species that is markedly different is T. sillamontana, native to Mexico, which has very short, succulent leaves, andisdensely coveredwithlongwhite hairs.
Aside from ornamental uses, leaves of several temperate species of Commelina and Tradescantia are edible. Historically, the Dakota Indians of the northern plains of the United States ate the young, spring shoots of T. occidentalis.
Other genera in the Commelinaceae, such as Zebrina, Rhoeo, Setcreasea, Cyanotis and Chocliostema, are tropical in origin, but grown in North America as ornamental house plants. In most of these tropical genera, there is a remarkable similarity in the appearance of flowers. For example, flowers in the genera Setcreasea, Rhoeo and Zebrina have flowers that look like miniature versions of those of Tradescantia.Most of these species have a sprawling growth form, and they readily root at nodes which are in contact with the soil. Most of these plants have stems and lower surfaces of leaves that are purple colored, due to the presence of pigments called anthocyanins. The combination of interestingly colored foliage, attractive flowers, and ease of propagation make these tropical species of spiderworts popular as indoor ornamentals.
These species may also have a other, relatively minor utilitarian values. For instance, Zebrina pendular is often used in introductory botany classes to demonstrate the size of plant vacuoles, and to show the presence of certain mineral structures of plants. In
Anthocyanin —A chemical pigment which is stored in the vacuoles of plant cells. Anthocyanins are red, purple, or blue in color, and may function to protect the plant from ultraviolet radiation, or to color flowers to attract bees.
Axil —An angular pocket formed on the upper surface of a leaf or petole where it is attached to a stem. Structures such as branch and inflorescence buds are commonly found in axils, and are referred to as axillary buds.
Locule —A hollow chamber within an organ. In plants, a locule is typically a chamber within an ovary or an anther.
Raphide —A needle shaped crystal, typically of calcium oxalate. Raphides are used as taxonomic characters in some plant families.
Terminal cyme —A shape of inflorescence that develop sbecause the terminal meristem becomes specialized to generate flowers. In the cyme, flowers open from the base of the inflorescence, sequentially tothe outermost flower.
Zebrina, vacuoles contain long needle-like crystals of calcium oxalate, called raphides, which are easily identified using a microscope. These vacuoles also contain the anthocyanin pigments that give the plant a purple color. Also, the leaf epidermal cells of Zebrina are large and easily removed from the rest of the leaf. These various features make Zebrina a good plant for demonstration purposes.
See also Horticulture.
The American Horticultural Society. The American Horticultural Society Encyclopedia of Plants and Flowers. New York: DK Publishing, 2002.
Stephen R. Johnson