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A group of small trees and shrubs, consisting of about two dozen species, confined to the Old World. Two are found in Europe, one in the Himalaya, the others in E. Asia as far south as the Chinese province of Yunnan. The cultivated species are deciduous, and have opposite leaves, usually neither toothed nor lobed; but in one species they are pinnate (S. pinnatifolia), and in another pinnately lobed (S. laciniata). The flowers appear in panicles, often pyramidal, but sometimes of indeterminate shape. Corolla tubular, with four lobes; calyx bell-shaped, unevenly toothed; stamens two. Seed-vessel a capsule of flattened or spindle shape, composed of two valves, which split from the top downwards when ripe.
There are two distinct subgenera:
subgen. Ligustrina. – Corolla-tube short (about as long as the calyx), white or cream-coloured; stamens clearly exserted. This group, which in some respects resembles the privets (Ligustrum), is composed of only two species: S. pekinensis and S. reticulata.
subgen. Syringa. – Corolla tube long, enclosing the stamens. Flowers usually in some shade of purple, sometimes white.
This subgenus is subdivided into four series by Rehder:
ser. Villosae. – The species of this well-marked group form a terminal bud and flower on the leafy shoots of the year.
ser. Syringa (Vulgares). – In this group, of which the common lilac is the type, the flowers are produced in leafless panicles from upper axillary buds on the previous season's growth and the terminal bud is usually lacking. Leaves simple or pinnately divided, glabrous or nearly so.
ser. Pinnatifoliae. – The one species in this series is closely allied to the preceding, but has completely pinnate foliage and terminal growth-buds.
ser. Pubescentes. – This group resembles the series Syringa in the manner in which the flowers are borne and in the absence of terminal buds, but in other respects approaches the series Villosae. The type of the group is S. pubescens, and the other species recognised are all closely allied to it.
The standard work on the genus is: Susan D. McKelvey, The Lilac (1928). It is profusely illustrated and treats the garden varieties and hybrids as well as the species. The author was for many years a Research Associate at the Arnold Arboretum, Massachusetts.
The cultivation of the common lilac is dealt with under the heading of S. vulgaris, and the soil and general treatment are the same for the rest of the genus. All of them can be propagated by layers, most of them by cuttings. Cuttings should be made of mature shoots in August, and placed in a sheltered position under handlights. Softer cuttings taken earlier will often take root in gentle heat.
The lilacs are subject to a bacterial disease, which causes the young shoots and inflorescences to wilt and blacken, and is often taken to be the result of frost damage. The affected parts should be cut out immediately the attack is noticed.
Botanical section revised by P. S. Green of The Herbarium, The Royal Botanic Gardens, Kew.