A 3D computer model of a Crack Willow (Salix fragilis) growing on the bank of a river. The Crack Willow is a largish tree typically 10-20 meters tall but sometimes 25-30 meters tall with spreading branches growing at angles of 60 to 90 degrees and prefers wet soils with its roots able to grow well in water. The Crack Willow is so-called because of its tendency to fragment and drop twigs and branches into the water. The twigs of willows in general will root if planted and produce new trees and the branches and twigs of willow may lodge downstream and root in the bank, forming new trees. This is a mode of asexual reproduction and dispersal that the crack willow is particularly good at. Indeed, sometimes whole trunks split apart. To prevent such dropping of branches, willows are often pollarded, though pollarding of trees can itself cause mechanical failure of tree trunks. In pollarding the tree is cut back to about 2 meters above ground and it responds by producing more smaller branches, an adaptive response to browsing and wind-damage, however, if these branches are left to grow too large the crown may become top-heavy and overload the trunk which may split, as has happened in some old oak pollards, but crack willow is especially prone to fragmentation as this assists its reproduction and colonisation of river banks.
The Crack Willow often has an irregular, leaning crown.
a diagrammatic cross-section (transverse section, T.S.) through
a young submerged root of Salix
Willow's preference for water means that many of roots will be
growing in waterlogged soils or even directly into water. This
poses the problem of how to get oxygen in to the root cells.
Many plants respond to water-logging by developing air-spaces
within the root cortex (the cortex is the parenchyma tissue between the
epidermis and the vascular cylinder) which are continuous with
the normal air spaces in the shoots above ground. This allows
the aerial shoots to supply the roots with air. This spongy,
aerated parenchyma tissue is called aerenchyma. Being adapted for wet
conditions, willows like the crack willow excel in
(schizogenous). In either case the response is a coordinated
one. Cells in the root that lack (schizogenous). In either case
the response is a coordinated one. Cells in the root that lack
oxygen produce ethylene (a plant hormone or phytohormone) which,
in the lysogenous case, triggers the synthesis of cellulase, an
enzyme that degrades the cellulose cell walls of parenchyma
cells, resulting in their lysis.
Did you know? A willow root 114.5 feet in length was once found blocking a storm sewer!
The bark is green and smooth when young, becoming rough and grey when older. The bark of willow trees the simplest type (also found in Magnolia, apple, pear and poplar) and consists of an opening filled with appearing later.)
The bark of willows contain high levels of the chemical salicylic acid (SA), a painkiller which was extracted as a forerunner to aspirin. Aspirin has fewer ill side-effects and can be easily made from salicylic acid. Another related compound, found in the wintergreens, is Oil of Wintergreen (methyl salicylate), also used as a painkiller and antiseptic. SA is a plant hormone with multiple functions, including activation of plant pathogen defense systems by triggering production of antimicrobial proteins. It can also trigger whole-plant protection in response to a localised infection and by conversion to volatile methyl salicylate it can pass this signal on to neighbouring plants and so function as an alarm signal. SA possibly has some antimicrobial properties of its own too, since it is an effective treatment for warts, suggesting anti-viral properties or some ability to prevent abnormal cell proliferation and possibly protects the plant against tumour formation (as induced by many pathogens). Aspirin has been shown to have anti-cancer properties.
Despite the fragility of the whole tree, the wood of Crack Willow hybrids is used to make cricket bats. The wood is pale cream to pink-brown, soft and brittle. Other willow species are famous for the flexibility of their wood which makes them resistant to wind damage, though these willows still have a tendency to shed many living twigs which can root to produce new plants.
The wood of willow (Salix nigra) is (semi-)diffuse-porous nonstoried wood with uniseriate heterocellular rays (!). Such descriptions allow wood to be classified and the terms will now be explained. Hardwoods are divided into two main types: ring-porous and diffuse-porous (though some types are intermediate). In diffuse porous wood the early xylem vessels (those formed in Spring) are no larger or only slightly larger than those formed later in the year (though annual growth rings are still visible) and is found in trees like maple, horse chestnut and holly. Contrary to this is the ring-porous type of wood in which the early xylem vessels are much larger than the later vessels and form definite rings, forming rings of pores visible with a hand-lens in a cross-section. Ring-porous trees include most oak tree species.
There are three ways to section a trunk. A transverse section or cross-section simply cuts across the long-axis, like removing a slice or neat log. Lengthwise sections, slicing the trunk along its height/axis may be of two types: radial sections cut through the centre of the trunk, along a radius, cutting it into two equal halves; tangential sections cut a lengthwise piece offset from the centre and hence cutting across the radii, but parallel to the long-axis (and parallel to a tangent to the outer circumference) and cutting the trunk into two unequal pieces.
The grains of different types of wood look different in these three sections. In radial section, rows of xylem vessels can be seen radiating from the centre to the outside of the trunk, but the wood is also formed from horizontal layers of cells piled vertically on top of one-another in tiers. These tiers are apparent in tangential sections. The appearance of the wood grain in tangential section divides wood into storied and nonstoried types according to how the tiers of cells stack together. In nonstoried wood, the grain simply runs smoothly along the wood as the cells of each tier overlap, so that the tiers appear to merge with one-another. In storied wood, the tiers do not intermingle and distinct layers can be seen.
The rays are vertical plates of parenchyma cells that extend along a radius of the trunk and these rays in particular affect the appearance of the grain in tangential section and in storied wood the rays are neatly aligned, such that the rays of one tier do not extend into the adjacent tiers. In nonstoried wood the rays are positioned irregularly with the rays of one tier penetrating into adjacent tiers, such that the separate tiers are not apparent but merge into one. Trees with storied wood include persimmon (Diospyros virginiana) and trees with nonstoried wood include: oak, willow, pecan (Carya pecan).
There are different types of rays in different species. Conifers tend to have rays that are rows of cells only one cell wide (uniseriate) whereas those of hardwoods may be either uniseriate or multiseriate (several cells wide). (The width and height of the rays can be best seen in tangential section, their length in radial section and their height in radial section). Rays range in height from one to many cells (up to 3 cm or more).
Homocellular rays are composed of cells that all look alike as they are all aligned in the same direction (either upright or procumbent ('lying down') along the radius such that upright ray cells appear taller when the ray is seen end-on in tangential sections). In heterocellular rays, some of the ray cells are upright (vertical) and others are lying down (horizontal) and so appear to be of two different sizes in section. The entire wood may have only either homocellular (homogeneous wood) or heterocellular rays (heterogeneous wood) or a combination of the two (also heterogeneous wood).
The leaves are long and slender (7 - 15 cm long), bright green on their upper surface, paler beneath. The margins are serrated (jagged).
Willows are usually dioecious (meaning they have separate sexes with separate male and female trees) but occasionally monoecious (having both male and female flowers on the same tree). Willows are famous for their long furry female catkins that are pollinated primarily by insects and birds (this is unusual as most catkins are wind-pollinated). Each flower in the catkin produces a visible drop of nectar and blue tits are one of the prime pollinators that feed on this nectar. The male catkins are shorter, but still about 5 cm long and have golden anthers. The fruit is a capsule with plumed seeds that assist their dispersal by wind. Each flower is subtended by a bract, the catkin-scale (a tiny modified leaf). Each female flower has 1-2 nectaries. Each nectary is a modified extension at the base of each catkin-scale and may be found in both male and female flowers. Each male flower has 1 to 12 stamens. The ovary has s single chamber containing many ovules and there are two stigmas. There may be further bracts at the base of the catkin.
Willow is a deciduous hardwood of temperate regions and is very fast growing. Like many deciduous trees it follows an annual growth cycle. It over-winters in a dormant state which is broken when flower bud-burst occurs in Februrary. Flowering peaks in March and at the same time leaf-bud dormancy is broken and over Spring and Summer shoot growth is rapid. Growth slows in late summer and ends at the end of August. Leaves then begin to turn yellow and fall and by November the tree is bare and the buds are dormant. Some growth may continue inside the stem over winter in Salix fragilis, with a small amount of narrow phloem sieve tubes being produced. The Weeping Willow, Salix babylonica, grows very fast but is short-lived with a lifespan of 20-30 years.
Salix herbacea is a form of creeping dwarf willow and the smallest willow in the British Isles, growing to 5 to 10 cm in height! It is adapted to cold montainous climates mostly in the Scottish Highlands and the Lake District. It forms an underground network of tough rhizomes that form a mat. This miniaturisation of trees also occurs in Arctic environments (typically such species are Arctic-montane, growing at high altitudes at lower latitudes and growing low altitudes at high latitudes, as the two environments are very similar) and the tiny Arctic Willow, Salix arctica, is found farther North than any other woody plant, growing far North of the treeline and at the edge of the land on Greenland. Miniaturisation means that the trees can cling to the ground, in the boundary layer of more still air, and so avoid the harsh, cold and drying winds higher up.
Willows (Salix) belong to the family Salicaceae along with several other genera including Populus (poplars and aspen). There are some 68 forms of Salix recognised in the British isles alone, of which 20 or so are hybrids, sometimes between three species.
Willows and poplars have characteristic glands on their leaves. These glands may occur at the leaf margins, with a gland on each leaf tooth and at the base of the leaf and on the leaf stalk and on the stipules (stipules are small leaf-like appendages which usually occur as a pair at the base of a leaf stalk). In at least some cases these glands are known to function as extrafloral nectaries, exuding drops of sugary fluid. In other cases they secrete resin: a sticky viscous fluid which solidifies when dry and may even be secreted as solid filaments. Secretion of solid resins may give plant parts a 'mealy appearance'. In some cases the glands may be hydathodes: water-secreting glands which may enable mineral transport in the xylem to continue in the absence of transpiration. These glands may be hemispherical or conical domes. Those at the base of the leaf may be borne on appendages. A gland from Salix lucida is illustrated below, in section (see: Curtis and Lersten, 1980):
This particular gland type occurs on the leaf teeth, the margins of the stipules and the bases of the leaves and stipules. It consists of a raised dome of modified epithelial cells, supported by 5 to 6 rows of isodiametric (having cuboidal symmetry, i.e. all three radii equal) parenchyma cells. The modified epithelium are very elongated and column-like and packed closely together like a palisade. One, sometimes several, vascular bundle supplies the gland with the water and raw materials it needs to synthesise its secretion. The elongated cells have additional volume to accommodate the synthetic machinery. The secretion accumulates as a viscous liquid, in young glands, between the secretory epithelium. In older glands, a circular pore forms in the cuticle in the centre of the gland and the resin is extruded, sometimes as more-or-less solid filaments. The oldest glands are exhausted and somewhat shrivelled and wrinkled with a sunken apex. The glands and glandular secretions of plants is a fascinating topic and the various secretions have many different functions, and in some cases the function is unknown.
Tree of Mystics and Poets
The association of willow with water and hence with the Moon has inspired generations of poets, artists
and mystics. For example see the artwork of Arthur Wrackham, whose work has appeared in many
publications, including The Wind in the Willows by Kenneth Grahame and the beautiful work of Glennie
Kindred. The upright mass of twigs so characteristic of old willow trees give them the appearance of
ancient beings and so they have been likened to old crones, witches and Moon magic and the Moon
goddess of the Underworld, Hecate.
The game of cricket requires a wooden bat, and the best choice of tree for the best bats is a variety of the white willow, Salix alba. This variety grows best only in parts of England, although it has been exported elsewhere, the success of this is limited. The best bats apparently still come from those trees grown in parts of England. In particularly, the properties of the wood make a bat that is resilient and durable and which strikes the ball with the correct amount of elastic recoil. The trees are nurtured from sapling to give the right quality of wood, and some parts make the best bats, whilst others make cheaper and lesser bats, though still of relatively good quality. The wood does not split easily, for the white willow has supple wood (when at the right age and maturity). This is somewhat 'paradoxical' as some willows really do 'bend in the wind' rather than fragmenting as do mature crack willows. The wood is soft and has to be compressed for use in bats. The blade of the bat is made of a single piece of wood and the spacing of the wood determines the durability of the bat. A good bat should make several centuries, a lesser bat perhaps only one.
Above: a young willow tree (Salix spp.). determining species of Salix is tricky, especially when the tree has yet to put out leaves as in this specimen seen in early Spring. Below: the bark has characteristic diamond-shaped lenticels and is developing reddish fissures.
Above and below: female catkins from the same tree. Willows are dioecious with separate male and female trees. this tree was flowering in March, which rules out Crack Willow, Salix fragilis which flowers at the time its first leaves expand, around April and May. Crack Willow is itself an aggregate of several 'varieties'.
Below:male catkins on a second tree.
Article updated: 29 April 2017; 1 April 2020.