Eucalyptus is a genus of over seven hundred species of flowering trees, shrubs or mallees in the myrtle family, Myrtaceae. Along with several other genera in the tribe eucalypteae, including Corymbia, they are commonly known as eucalypts. Plants in the genus Eucalyptus
have bark that is either smooth, fibrous, hard or stringy, leaves with oil glands, and sepals and petals that are fused to form a "cap" or operculum over the stamens. The fruit is a woody capsule commonly referred to as a "gumnut".
Eucalypts vary in size and habit from shrubs to tall trees. Trees usually have a single main stem or trunk but many eucalypts are mallees that are multistemmed from ground level and rarely taller than 10 metres (33 feet). There is no clear distinction between a mallee and a shrub but in eucalypts, a shrub is a mature plant less than 1 m (3 ft 3 in) tall and growing in an extreme environment.
Eucalyptus trees, including mallees and marlocks, are single-stemmed and include Eucalyptus regnans, the tallest known flowering plant on Earth.
All eucalypts add a layer of bark every year and the outermost layer dies. In about half of the species, the dead bark is shed exposing a new layer of fresh, living bark. The dead bark may be shed in large slabs, in ribbons or in small flakes.
Nearly all eucalyptus are evergreen, but some tropical species lose their leaves at the end of the dry season. As in other members of the myrtle family, eucalyptus leaves are covered with oil glands. The copious oils produced are an important feature of the genus. Although mature eucalyptus trees may be towering and fully leafed, their shade is characteristically patchy because the leaves usually hang downwards.
Eucalyptus features in a range of preparations to relieve symptoms of the common cold, for example, cough lozenges and inhalants.
Herbal remedies recommend using fresh leaves in a gargle to relieve a sore throat, sinusitis and bronchitis. Also, eucalyptus oil vapor appears to act as a decongestant when inhaled. It is a popular home remedy for colds and bronchitis.