1911 Encyclopædia Britannica/Willow
WILLOW (Salix), a very well-marked genus of plants constituting, with the poplar (Populus), the order Salicaceae. Willows are trees or shrubs, varying in stature from a few inches, like the small British S. herbacea and arctic species generally, to 100 ft., and occurring most abundantly in cold or temperate climates in both hemispheres, and generally in moist situations; a few species occur in the tropical and sub-tropical portions of the three great continents. Their leaves are deciduous, alternate, simple, and generally much longer than broad, whence the term willow-leaved has become proverbial. At their base they are provided with stipules, which are also modified to form the scales investing the winter buds. The flowers are borne in catkins (fig. 1), which are on one tree male (staminate) only, on another female (pistillate). Each male flower consists of a small scale or bract, in the axil of which are usually two, sometimes three, rarely five stamens, and still more rarely a larger number. In addition there is a small glandular disk, which assumes different shapes in different species. The female flowers are equally simple, consisting of a bract, from whose axil arises usually a very short stalk, surmounted by two carpels adherent one to the other for their whole length, except that the upper ends of the styles are separated into two stigmas. When ripe the two carpels separate in the form of two valves and liberate a large number of seeds, each provided at the base with a tuft of silky hairs, and containing a straight embryo without any investing albumen. The flowers appear generally before the leaves and are thus rendered more conspicuous, while passage of pollen by the wind is facilitated. Fertilization is effected by insects, especially by bees, which are directed in their search by the colour and fragrance of the flowers; but some pollen must also be transported by the wind to the female flowers, especially in arctic species which, in spite of the poverty of insect life, set abundant fruit. The tuft of hairs at the base facilitates rapid dispersion of the seed, early germination of which is rendered desirable owing to its tenuity. Although the limitations of the genus are well marked, and its recognition in consequence easy, it is otherwise with regard to the species. The greatest difference of opinion exists among botanists as to their number and the bounds to be assigned to each; and the cross-fertilization that takes place between the species intensifies the difficulty. Andersson, a Swede, spent nearly a quarter of a century in their investigation, and ultimately published a monograph which is the standard authority on the subject. He admits about a hundred species. Professor C. S. Sargent (Silva of North America) suggests 160 to 170 as the number of distinguishable species. Some botanists have enumerated 80 species from Great Britain alone, while others count only 12 or 15. Dr Buchanan White, who made a special study of the British willows, grouped them under 17 species with numerous varieties and hybrids. To illustrate the great perplexity surrounding the subject, we may mention that to one species, S. nigricans, one hundred and twenty synonyms have been attached. Some of these are doubtless such as no botanist, with adequate material for forming an opinion, would accept; but, after making the necessary deductions for actual mistakes and misstatements, there still remains a large number upon which legitimate differences of opinion prevail. Andersson says that he has rarely seen two specimens of this species which were alike in the collective characters offered by the stature, foliage and catkins. No better example could be found of the almost limitless variation in so-called species.
Fig. 1.—Salix caprea—Common Sallow or Goat Willow.
|1. Leaf shoot.
2. Branchlet bearing male catkins.
3. Male flower.
|4. Female catkin. |
5. Female flower.
6. Capsule, opened.
1, 2, 4 reduced; 3, 5-7 enlarged.
Fig. 2.—Salix fragilis—Crack Willow.
|A, Flowering shoot from male plant
B, Flowering shoot from female plant
2, Catkin of fruits.
3, Male flower.
|4, Female flower with and without bract. |
5, Single fruit from which the hairy seeds are escaping; one seed shown separately.
A, B, 1, 2, about half nat. size, 3-5 enlarged.
Few genera have greater claims to notice from an economic point of view. As timber trees many of the species are valuable from their rapidity of growth and for the production of light durable wood, serviceable for many purposes. Among the best trees of this kind are S. fragilis, the crack willow (fig. 2), especially the variety known as S. fragilis, var. Russelliana and S. alba, the white or Huntingdon willow. These trees are usually found growing by rivers' banks or in other moist situations, and are generally pollarded for the purpose of securing a crop of straight poles. This plan is, however, objectionable, as inducing decay in the centre of the trunk. Where poles are required, it is better to treat the trees as coppice and to cut the trunk level with the soil. The wood of S. fragilis is used for cricket-bats; there is a great difference in the value for this purpose of timber from different soils; and wood of the female tree is said to be preferable to that of the male. S. caprea (fig. 1), a hedgerow tree, generally grows in drier situations. It is a useful timber tree, and its wood, like that of S. alba, is prized in the manufacture of charcoal. Its catkins are collected in England in celebration of Palm Sunday, the bright-coloured flowers being available in early spring when other decorations of the kind are scarce. Certain sorts of willow are largely used for basket-making and wicker-work. The species employed for this purpose are mostly of shrubby habit, and are known under the collective name of osiers (see Basket, and Osier). The best for planting is the bitter osier, S. purpurea; planted on rich, well-drained soil, subject to occasional immersion, this willow may be grown profitably for basket-work. It is also well adapted for forming wind-breaks or screens, or for holding the banks of streams and preventing the removal of the soil by the current. S. viminalis is one of the best of the green osiers, suitable for hoops and valuable for retaining the soil on sloping embankments. S. vitellina yields the yellow osiers. S. acuminata and other species do well by the seaside, and are serviceable as wind-screens, nurse-trees and hedges. S. daphnoides, S. repens and other dwarf kinds are useful for binding heathy or sandy soil. In addition to their use for timber or basket-making, willows contain a large quantity of tannin in their bark. A valuable medicinal glucoside named salicin (q.v.) is also extracted from the bark. The wood, especially of S. alba, is used for paper pulp. As ornamental trees some willows also take a high rank. The white willow is a great favourite, while the drooping habit of the weeping willow renders it very attractive. Though named S. babylonica, it is really a native of China, from which it has been widely spread by man; the willow of the Euphrates (Ps. cxxxvii.) is in all probability Populus euphratica. S. babylonica is sometimes spoken of as Pope's willow, having been cultivated by that poet, or as Napoleon's willow, because his tomb at St Helena is overshadowed by a tree of this species, from which many offsets exist or are reputed to exist in modern gardens. S. regalis has very white, silvery leaves. S. rosmarinifolia is remarkable for its very narrow leaves—purplish above, silvery beneath.
The larvae of several nocturnal Lepidoptera feed upon the leaves of the willows, and the trunk of the sallow is often injured by the perforations of the lunar hornet sphinx (Trochilium crabroniforme).