Description
Botanical drawing of the leaves, flowers and samaras from Britton and Brown's 1913 Illustrated flora of the northern states and Canada

A. altissima is a medium-sized tree that reaches heights between 17 and 27 metres (56 and 90 ft) with a diameter at breast height of about 1 metre (40 in).[2] The bark is smooth and light grey, often becoming somewhat rougher with light tan fissures as the tree ages. The twigs are stout, smooth to lightly pubescent, and reddish or chestnut in colour. They have lenticels as well as heart-shaped leaf scars (i.e. a scar left on the twig after a leaf falls) with many bundle scars (i.e. small marks where the veins of the leaf once connected to the tree) around the edges. The buds are finely "pubescent", dome shaped, and partially hidden behind the petiole, though they are completely visible in the dormant season at the sinuses of the leaf scars.[3] The branches are light to dark gray in colour, smooth, lustrous, and containing raised lenticels that become fissures with age. The ends of the branches become pendulous. All parts of the plant have a distinguishing strong odour that is often likened to rotting peanuts or cashews.[4]

The leaves are large, odd- or even-pinnately compound, and arranged alternately on the stem. They range in size from 30 to 90 cm (1 to 3 ft) in length and contain 10-41 leaflets organised in pairs, with the largest leaves found on vigorous young sprouts. The rachis is light to reddish-green with a swollen base. The leaflets are ovate-lanceolate with entire margins, somewhat asymmetric and occasionally not directly opposite to each others. Each leaflet is 5 to 18 cm (2 to 7 in) long and 2.5 to 5 cm (1 to 2 in) wide. They have a long tapering end while the bases have two to four teeth, each containing one or more glands at the tip.[3] The leaflets' upper sides are dark green in colour with light green veins, while the undersides are a more whitish green. The petioles are 5 to 12 mm (0.2 to 0.5 in) long.[4] The lobed bases and glands distinguish it from similar sumac species.
Bark and flowers of A. altissima

Immature seeds on a female tree.

The flowers are small and appear in large panicles up to 50 cm (20 in) in length at the end of new shoots. The individual flowers are yellowish green to reddish in colour, each with five petals and sepals.[2][4] The sepals are cup-shaped, lobed and united while the petals are valvate (i.e. they meet at the edges without overlapping), white and hairy towards the inside.[3][5][6] They appear from mid-April in the south of its range to July in the north. A. altissima is dioecious, with male and female flowers being borne on different individuals. Male trees produce three to four times as many flowers as the females, making the male flowers more conspicuous. Furthermore, the male plants emit a foul smelling odour while flowering to attract pollinating insects. Female flowers contain ten (or rarely five through abortion) sterile stamens (stamenoides) with heart-shaped anthers. The pistil is made up of five free carpels (i.e. they are not fused), each containing a single ovule. Their styles are united and slender with star-shaped stigmas.[3][5] The male flowers are similar in appearance, but they of course lack a pistil and the stamens do function, each being topped with a globular anther and a glandular green disc.[3] The seeds borne on the female trees are 5 mm in diameter and each is encapsulated in a samara that is 2.5 cm long (1 in) and 1 cm (0.4 in) broad, appearing July though August, but usually persisting on the tree until the next spring. The samara is twisted at the tips, making it spin as it falls and assisting wind dispersal.[2][4] The females can produce huge amounts of seeds, normally around 30,000 per kilogram (14,000/lb) of tree.[2]

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Taxonomy

The first scientific descriptions of the tree of heaven were made shortly after it was introduced to Europe by the French Jesuit Pierre Nicholas d'Incarville. d'Incarville had sent seeds from Peking via Siberia to his botanist friend Bernard de Jussieu in the 1740s. The seeds sent by d'Incarville were thought to be from the economically important and similar looking Chinese varnish tree (Toxicodendron vernicifluum), which he had observed in the lower Yangtze region, rather than the tree of heaven. d'Incarville attached a note indicating this, which caused much taxonomic confusion over the next few decades. In 1751, Jussieu planted a few seeds in France and sent others on to Philip Miller, the superintendent at the Chelsea Physic Garden, and to Philip C. Webb, the owner of an exotic plant garden in Busbridge, England.[3]

Confusion in naming began when the tree was described by all three men with three different names. In Paris, Linnaeus gave the plant the name Rhus succedanea, while it was known commonly as grand vernis du Japon. In London the specimens were named by Miller as Toxicodendron altissima and in Busbridge it was dubbed in the old classification system as Rhus Sinese foliis alatis. There are extant records from the 1750s of disputes over the proper name between Philip Miller and John Ellis, curator of Webb's garden in Busbridge. Rather than the issue being resolved, more names soon appeared for the plant: Jakob Friedrich Ehrhart observed a specimen in Utrecht in 1782 and named it Rhus cacodendron.[3]

Light was shed on the taxonomic status of ailanthus in 1788 when René Louiche Desfontaines observed the samaras of the Paris specimens, which were still labelled Rhus succedanea, and came to the conclusion that the plant was not a sumac. He published an article with an illustrated description and gave it the name Ailanthus glandulosa, placing it in the same genus as the tropical species then known as A. integrifolia (white siris, now A. triphysa). The name is derived from the Ambonese word ailanto, meaning "heaven-tree" or "tree reaching for the sky".[7][3] The specific glandulosa, referring to the glands on the leaves, persisted until as late as 1957, but it was ultimately made invalid as a later homonym at the species level.[3] The current species name comes from Walter T. Swingle who was employed by the United States Department of Plant Industry. He decided to transfer Miller's older specific name into the genus of Desfontaines, resulting in the accepted name Ailanthus altissima.[8] Altissima is Latin for "very tall", and refers to the sizes the tree can reach. The plant is sometimes incorrectly cited with the specific epithet in the masculine (glandulosus or altissimus), which is incorrect since botanical, like Classical Latin, treats most tree names as feminine.

There are three varieties of A. altissima:
A. altissima var. altissima, which is the type variety and is native to mainland China.
A. altissima var. tanakai, which is endemic to northern Taiwan highlands. It differs from the type in having yellowish bark, odd-pinnate leaves that are also shorter on average at 45 to 60 cm (18–24 in) long with only 13-25 scythe-like leaflets.[9][10][11] It is listed as endangered in the IUCN Red List of threatened species due to loss of habitat for building and industrial plantations.[12]
A. altissima var. sutchuenensis, which differs in having red branchlets.[9][10]