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Elm

disease trees elms water

Elms are trees (occasionally shrubs) of flowering plants in the genus Ulmus. Elm leaves possess stipules, and often have a nonsymmetrical leaf, that is, one half is larger than the other so that the bottom ends do not meet where they are attached to the mid-rib. Elms flower in the spring. Their flowers lack petals, form reddish brown clusters in the tops of the trees, appear before the leaves have fully expanded, and are pollinated by the wind. The fruits are samaras that are technically equivalent to the keys (fruits) of maple, although in elms the seed is surrounded entirely by a greenish, papery wing so that the oval or circular fruit (seed plus wing) resembles a fried egg.

There are about 30 species of elms in the world. Most occur in north temperate regions of North America, Europe, and Asia, and on mountains of tropical Asia. Elms rarely occur in large tracts in forests. Instead they are usually found interspersed among other deciduous trees.

Elm has, until recently, been an important ornamental and shade tree. In eastern North America there is hardly a single town without an Elm Street, named after the stately white or American elm (Ulmus americana), which is a tall tree reaching heights of 130 ft (40 m). American elm has elegant upswept limbs that arch down at the ends. Unfortunately, these beautiful trees are becoming rare because of a devastating disease called Dutch elm disease that was accidentally imported from Europe, probably on infected timber.

Dutch elm disease derives its name from the fact that it was first discovered on elm trees in Holland in 1921. There is no tree known as Dutch elm, and the disease originated in Asia. The disease appeared in the United States in 1930 and spread rapidly throughout the range of elm in eastern North America.

Dutch elm disease is caused by an ascomycete fungus (Ophiostoma ulmi) in partnership with an insect. Although the fungus causes the disease, an insect is generally necessary as an agent in spreading fungal spores from infected trees to healthy trees. The bark beetles Scolytus multistriatus, S. scolytus, and Hylurgopinus rufipes are the common agents of spread, although birds and, to a limited extent, water and wind can also spread the disease. The fungus can also spread from diseased trees to healthy trees by natural root grafts. Once a tree has been infected, the fungus initially grows within the water-conducting cells called vessels, where spores (different from those produced on coremia) are produced and released. Carried in the water of the vessels, these spores spread the infection to other parts of the tree. The symptoms shown by infected trees include: yellowing of the leaves, followed by wilting and their premature fall, death of branches causing the crown to appear sparse, and browning of the sapwood. If the infection spreads throughout the vascular system, death of the tree can occur within weeks of the infection, although many trees survive for several years before succumbing to the disease. In the later stages of the disease, the fungus moves out of the water-conducting cells into other tissues. There is no satisfactory method for managing Dutch elm disease. Not all species of elm have been affected by Dutch elm disease. Asiatic species, such as Siberian and Chinese elms, are generally resistant.

Elm wood is economically valuable. Most species produce fine timber with a distinctive pattern. The timber resists decay when waterlogged, thus making it quite useful in certain specialized uses, such as serving as underwater pilings. Before metalworking, elm was used in Europe in water pipes and water pumps; 200-year-old pipes are often dug up in London, England. The grain of elm wood is strongly interlocked so that it is very difficult to split. For this reason, elm wood is often used for certain kinds of furniture, such as the seats of chairs, since the driving in of legs and backs tends to split most other woods, and also for wheel-hubs and mallet heads. Elm wood has also been extensively used for coffin boards and shipping cases for heavy machinery.

The inner bark of elm, especially of roots, is fibrous and can be made into rope, and string for fishing-line, nets, or snares. The fruits are eaten by many birds and squirrels. Twigs and leaves are eaten by deer and rabbits. The leaves are quite nutritious and in ancient times in Europe, branches were cut off so that livestock could feed on the foliage. Elms have little value as food for people, although in times of famine, the ground bark, leaves, and fruits have been eaten by the Chinese, and bark ground into a meal and mixed with flour for bread has similarly been used in times of scarcity in Norway. The inner bark of slippery elm reputedly has some medicinal properties.

Les C. Cwynar

KEY TERMS

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Samara

—A simple, dry, fruit that is unopen at maturity, usually contains one seed, and has one or more wings.

Stipule

—An appendage found at the base of a leaf where it joins a branch or stem.

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