Tilia is a genus of about 30 species of trees native throughout most of the temperate Northern Hemisphere. They are generally called lime in Britain and linden or basswood in North America. The greatest species diversity is found in Asia, and the genus also occurs in Europe and eastern North America, but not western North America. Under the Cronquist classification system, this genus was placed in the family Tiliaceae, but genetic research by the Angiosperm Phylogeny Group has resulted in the incorporation of this family into the Malvaceae.
Tilia species are large deciduous trees, reaching typically 20 to 40 metres (66 to 130 ft) tall, with oblique-cordate leaves 6 to 20 centimetres (2 to 8 in) across. The exact number of species is subject to considerable uncertainty, as many or most of the species will hybridise readily, both in the wild and in cultivation.
"Lime" is an altered form of Middle English lind, in the 16th century also line, from Old English feminine lind or linde, Proto-Germanic *lendā, cognate to Latin lentus "flexible" and Sanskrit latā "liana". Within Germanic languages, English "lithe", German lind "lenient, yielding" are from the same root.
"Linden" was originally the adjective, "made from lime-wood" (equivalent to "wooden"); from the late 16th century, "linden" was also used as a noun, probably influenced by translations of German romance, as an adoption of Linden, the plural of German Linde. Neither the name nor the tree is related to the citrus fruit called "lime" (Citrus aurantifolia, family Rutaceae). Another common name used in North America is basswood, derived from bast, the name for the inner bark (see Uses, below). In the US, the name "lime" is used only for the citrus tree. Teil is an old name for the lime tree.
Latin tilia is cognate to Greek πτελέᾱ, ptelea, "elm tree", τιλίαι, tiliai, "black poplar" (Hes.), ultimately from a Proto-Indo-European word *ptel-ei̯ā with a meaning of "broad" (feminine); perhaps "broad-leaved" or similar
The Tilia's sturdy trunk stands like a pillar and the branches divide and subdivide into numerous ramifications on which the twigs are fine and thick. In summer, these are profusely clothed with large leaves and the result is a dense head of abundant foliage
The leaves of all the Tilia species are heart-shaped and most are asymmetrical, and the tiny fruit, looking like peas, always hang attached to a ribbon-like, greenish-yellow bract, whose use seems to be to launch the ripened seed-clusters just a little beyond the parent tree. The flowers of the European and American Tilia species are similar, except the American bears a petal-like scale among its stamens and the European varieties are devoid of these appendages. All of the Tilia species may be propagated by cuttings and grafting, as well as by seed. They grow rapidly in rich soil, but are subject to the attack of many insects
In particular, aphids are attracted by the rich supply of sap, and are in turn often "farmed" by ants for the production of the sap which the ants collect for their own use, and the result can often be a dripping of excess sap onto the lower branches and leaves, and anything else below. Cars left under the trees can quickly become coated with a film of the syrup ("honeydew") thus dropped from higher up. The ant/aphid "farming" process does not appear to cause any serious damage to the trees.