Abobe: a diagrammatic cross-section (transverse section, T.S.) through a young submerged root of Salix
fragilis. 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
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.
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 metres tall but sometimes 25-30 metres 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 metres 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.
Flowers and Fruit
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.
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
margines are serrated (jagged).
The 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.
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):
Bibliography / References
- P. Thomas, 2000. Trees: Their natural history. Cambridge University Press.
- K. Esau, 1976. Anatomy of seed plants, 2nd ed. Wiley (pub).
- M. Kawase and R.E. Whitmoyer, 1980. Aerenchyma development in waterlogged plants. Am. J.
Bot. 67: 18-22.
- R. Alvim, E.W. Hewett and P.F. Saunders, 1976. Seasonal variation in the hormone content of
Willow: I. Changes in abscisic acid content and cytokinin activity in the xylem sap. Plant Physiol. 57:
- C. Hart and C. Raymond, 1973. British trees in Colour. Book Club Associates (pub).
- J.R. Lawton, 1976. Seasonal variation in the secondary phloem from the main trunks of Wllow and
Sycamore trees. New Phytologist 77: 761-771.
- W.J. Bean, 1907. The Cricket Bat Willow. Bulletin of Miscellaneous Information (Royal Gardens,
Kew) 7: 311-316.
- Meikle, R.D. 1984. Willows and Poplars of Great Britain and Ireland. BSBI Handbook No 4. BSBI.
- Curtis, J.D. and N.R. Lersten, 1980. Morphology and anatomy of resin glands in Salix lucida
(Salicaceae). American Journal of Botany 67: 1289-1296.
The Crack Willow often has an irregular, leaning crown.
- Glennie Kindred,1995. The Sacred Tree. ISBN: 0-9532227-1-3.
- Glennie kindred, 1997. The Tree Ogham. ISBN: 0-9532227-2-1.
- J.M. Paterson, 1996. Tree Wisdom: The definitive guidebook to the myth, folklore and healing
power of trees. Thorsons (pub).
- J. Gifford, 2000. The Celtic Wisdom of trees: Mysteries, magic and medicine. Godsfield Press Ltd.
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 amde 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.
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.
Article updated: 29 April 2017.