Arum Family (Araceae)
Arums, also called aroids, are flowering plants in the family Araceae. The 2,500 species of arums are distributed worldwide, primarily in tropical and subtropical regions, where they grow in rainforests, mostly on the ground but also commonly as epiphytes. Arums are generally absent from the arctic and deserts. Only 11 species occur in North America and other north temperate regions.
Most species are small to medium-sized perennial herbs, often climbing as vines, and a few are shrubs. The leaves of arums are generally broad, frequently dissected, and occasionally have natural holes. The leaves commonly contain abundant sharp-pointed crystals of calcium oxalate, which give the leaves an acrid smell. Calcium oxalate crystals are poisonous and irritating when chewed, thus protecting the leaves from herbivores. The tiny flowers are densely borne on a stucture called a spadix, which is accompanied by a large, often colorful leaf known as a spathe. The spathe varies in size and shape, but in the most advanced species of arums it forms a hood that encloses the spadix, as in the familiar jack-in-the-pulpit (Arisaema triphyllum) of North America. The fruits of arums are almost always brightly colored berries that are eaten and dispersed by animals.
Arums are famous for the variety of offensive odors they produce in association with a pollination strategy that involves deception. Arums are usually pollinated by flies or beetles that normally feed on rotting organic matter, such as decaying plants or mushrooms, dung, or animal carcasses. Arums mimic the odors of decay by emitting vapors of fatty compounds from their spadix and spathe, thereby luring insects who expect to find a tasty mass of rotting flesh or decaying plants.
Many aroids imprison the insects that they deceive. A fine example of this strategy is Helicodiceros muscivorus, a native of Corsica, Sardinia, and nearby Mediteranean islands. On the island of Calvioli, this plant grows in open areas between rocks, sometimes in gull colonies. Bird droppings, regurgitated seafood, the carcasses of chicks, and eggs broken by predators all contribute to breathtaking odors. Helicodiceros flowers when the gulls are breeding, and it produces an open spathe with the shape of a shortened, slightly compressed bull-horn. The spathe is a mottled grey-and-red color, and its appearance and odor resemble rotting meat. Excited blowflies will actually choose the stench of the arum over the gull-mess, landing on the spathe in search for food. They are eventually drawn to the dark, smelly, narrow end of the spathe, where they enter through a small opening into a chamber, which becomes their dungeon. The blowflies are unable to escape because of a dense barrier of stiff, sharp, downward-pointing hairs that guard the opening. The chamber encloses the basal portion of the spadix, on which are located the flowers. Some of the flies will have previously visited other plants and been dusted by pollen. When the flies first become trapped, only the female flowers are receptive and so they will be pollinated by the accidental stumbling of the blowflies in the dungeon. The flowers also exude a small amount of nectar, just enough to keep the flies alive. After a few days, the male flowers mature, and release their pollen onto the blowflies. Simultaneously, the sharp hairs wither, thus releasing the flies. Some of the flies will be duped a second time and cross-pollinate the arums.
Arums are unique among plants in possessing a remarkable ability to generate metabolic heat. Their spadix commonly respires fat rapidly to produce heat during pollination, apparently as a means of increasing the vaporization of their foul-smelling compounds. The philodendron (Philodendron scandens) can raise its spadix temperature to as high as 116°F (47°C), even when the air is close to freezing. The skunk cabbage (Symplocarpus foetidus), which is a native of swamps and other wet places in eastern North America, flowers early in the spring, often when the ground is still covered by snow. Its spadix can generate enough heat to attain temperatures up to 77°F (25°C) above air temperatures, melt the surrounding snow, and get a head start on attracting flies.
The arum known as jack-in-the-pulpit is a perennial plant, native to moist or wet forests throughout eastern North America, and it is sometimes cultivated as an interesting garden ornamental. The sex of individual plants depends on their size. When small, they only produce male flowers, but in later years when they are larger, they switch sex and produce only female flowers. The explanation for this phenomenon appears to be that when the plant is small, it has relatively few resources available to it, insufficient to develop the large berries that the jack-in-the-pulpit produces. Therefore, plants are male first because pollen grains are small and take relatively little energy to produce. When the plant becomes larger, it can afford to invest in the higher costs of producing fruits. Thus, it switches its sex to female.
Several arums are important economically. The Monstera deliciosa produces an edible fruit and is a popular indoor plant because of its unusual leaves, which have large holes due to arrested development of parts of its growing surface. Colocasia esculentum, commonly called taro or poi, is a native of Asia with many varieties that are widely cultivated in the tropics because of their large, starch-rich tubers. Many arums are cultivated as ornamentals for their interesting foliage, which is often intricately dissected, such as the Philodendron and Monstera described previously. Other species are prized as indoor plants because of their large, brightly colored spathes, which may range in color from pure white to bright red and even iridescent. Species of the genus Cryptocoryne are commonly used as aquarium plants.
Brown, D. 1988. Aroids: Plants of the Arum Family. Portland, OR: Timber Press, 1988.
Dahlgren, R.M.T., H.T. Clifford, and P.F. Yeo. The Families of the Monocotyledons: Structure, Evolution, and Taxonomy. Berlin: Springer-Verlag, 1985.
Les C. Cwynar