Hedera helix is the commonest species of Hedera, Ivy, in the British Isles. It is widespread in woodlands but common also on cliffs, rock outcrops or man-made structures including brick or masonry walls and wooden structures. On level ground it has horizontal, creeping stems not exceeding 5–20 cm height, often growing through leaf litter and sending leaves upwards and roots downwards at the nodes. But when it encounters a vertical surface it can climb to at least 30 m above the ground. Ivies have two leaf types, with palmately lobed juvenile leaves on creeping and climbing stems and unlobed cordate adult leaves on fertile flowering stems exposed to full sun, usually high in the crowns of trees or the tops of rock faces, from 2 m or more above ground. The juvenile and adult shoots also differ: the former being slender, flexible and scrambling or climbing with small adventitious roots to fix the shoot to the substrate (rock or tree bark); the latter thicker, self-supporting and without roots. The flowers are greenish-yellow with five small petals; they are produced in umbels in autumn to early winter and are very rich in nectar. They are pollinated chiefly by hoverflies, Hymenoptera, and are particularly attractive to common wasps. The fruit is a berry, ripening in late winter to mid-spring. The seeds are dispersed by birds which eat the berries.
ETYMOLOGY and CULTURAL SYMBOLISM
The name Ivy derives from Old English ifig, cognate with German Efeu, of unknown original meaning (Onions, 1966). Hedera is the classical Latin name for the plant. Other names in Britain, now defunct, include “Bindwood” and “Lovestone”, for the way it clings to these substrates.
Evergreen plants have been seen as a harbinger of spring since pre-Christian times, and therefore involved in celebrations of the mid-winter solstice, as they still are. This is perhaps the origin of Ivy in mediaeval Christian symbolism. It was seen as a symbol of the eternal life of the soul, and as a symbol of the Blessed Virgin Mary (e.g. in the carol “The Holly and the Ivy”), and, perhaps for this reason, ivy used to be presented by priests to those whom they had just married (Hulbert, 1992). Ivy was brought into homes to drive out evil spirits. In Ancient Greece, wreaths of ivy were used to crown victorious athletes, and in Rome it was believed that a wreath of ivy could prevent a person from becoming drunk; an ivy wreath was worn by Bacchus, the god of intoxication. Perhaps it is for this reason that ivy-wrapped poles were traditionally used to advertise taverns in the United Kingdom, and many pubs are still called The Ivy.
Ivy-covered ruins were a staple of the Romantic movement in landscape painting, for example ‘Visitor to a Moonlit Churchyard’ by Philip James de Loutherbourg (1790), ‘Tintern Abbey, West Front’ by J.M.W. Turner (1794) and ‘Netley Abbey’ by Francis Towne (1809). In this context ivy may represent the ephemerality of human endeavours and the sublime power of nature.
Hedera L., commonly called ivy (plural ivies), is a genus of 12–15 species of evergreen climbing or ground-creeping woody plants, in the family Araliaceae, native to western, central and southern Europe, Macaronesia, north-western Africa and across central-southern Asia east to Japan and Taiwan. The Araliaceae is a mainly tropical family in the order Apiales, of 41-50 genera, c. 1450 spp. Its range is most of the southern hemisphere except central Australia, and parts of the northern hemisphere. It is absent from most of north Africa (Sahara) and from most of Asia except Turkey, the far east and Himalaya. It is absent from Arctic and sub-Arctic. Genera of the Araliaceae are small to medium trees, shrubs, climbers (Hedera) or herbs (Aralia). Some species have medicinal qualities: Panax spp. are the source of ginsengs (Heywood et al. 2007).
Three subspecies of Hedera helix L. are recognized in the Flora Europaea: ssp. helix, ssp. poetarum and ssp. canariensis. Hedera hibernica (Kirchner) Bean is the autotetraploid of H. helix, and therefore not interfertile with it. It is often included with H. helix sensu stricto as H. helix sensu lato. The two subspecies can be distinguished by the morphology of their trichomes. Despite its name, it is not certain that H. hibernica originated in Ireland, and it should not be confused with the horticultural cultivar, H. ‘Hibernica’!
The only verified hybrid involving ivies is the intergeneric hybrid ×Fatshedera lizei, a cross between Fatsia japonica and Hedera hibernica. This hybrid was produced once in a garden in France in 1910 and never successfully repeated, the hybrid being maintained in cultivation by vegetative propagation. Hybridisation may however have played a part in the evolution of some species in the genus.
Hedera species differ in detail of the leaf shape and size (particularly of the juvenile leaves), in the structure of the leaf trichomes, and also in the size and, to a lesser extent, the colour of the flowers and fruit. The chromosome number also differs between species. The basic diploid number of Hedera is 48 (including H. helix), while some (including H. hibernica) are tetraploid with 96, and others hexaploid with 144 and octaploid with 192 chromosomes.
DISTRIBUTION and HABITAT
Ivies are natives of Eurasia and North Africa but have been introduced to North America, where they invade disturbed forest areas, and Australia. H. helix is a plant of recent, not ancient, woods in England (Rackham (2015), p.258), but of ancient, not recent, woodlands in Poland (Zwarenko and Loster 1992). It likes phosphate-rich, fertile soil (Rackham (2015) p.178).
FLOWERS AND FRUITS
H. helix flowers September to November; fruits ripen thereafter. The flowers are pollinated chiefly by hoverflies and Hymenoptera, and are particularly attractive to common wasps. The ivy bee Colletes hederae is completely dependent on ivy flowers, timing its entire life cycle around ivy flowering (Hymettus-BWARS, 2011). The fruit is a berry that ripens during the winter and spring, and is blue/black when ripe. The fruit are eaten by a range of birds, including thrushes, blackcaps, and woodpigeons (Mitchell (1975).
Ivy is one of only 2 evergreens that are (moderately) palatable to livestock. Most spp. of deer like ivy. Slugs and snails reject ivy leaves (Metcalfe 2005). The fruits are eaten by a range of birds, including thrushes, blackcaps, and woodpigeons. The leaves are eaten by the larvae of some species of Lepidoptera such as angle shades, lesser broad-bordered yellow underwing, scalloped hazel, small angle shades, small dusty wave (which feeds exclusively on ivy), swallow-tailed moth and willow beauty.
Little lambs do not eat Ivy, despite the claim in the children’s song
Mairzy doats and dozy doats and liddle lamzy divey; A kiddley divey too, wouldn’t you?
The lyrics discombobulate to:
Mares eat oats and does eat oats and little lambs eat ivy; A kid will eat ivy too, wouldn’t you?.
WATER CONDUCTION IN XYLEM
Lianas and other climbing plants are characterized by large crowns supplied by long and flexible but narrow main stems (Schnitzer and Bongers, 2002; Wyka et al., 2013; Rosell and Olson, 2014). The discrepancy between leaf area and supporting stem cross-sectional area has consequences for both static and water-supply aspects (Ewers, 1985). On the one hand, the mechanical function of the stem, and thus resource allocation to the xylem, can be reduced, as climbers are not self-supporting but rely on neighbouring trees or other structures to support their weight.
At this point I must make a detour into the realm of physics: you may regard this as your Christmas treat. Enough water has to be brought to the canopy to satisfy the demands of transpiration of the crown foliage. Water transport and regulation, on the other hand, have to be optimized with regard to capacity and safety by a hydraulic architecture adapted to this growth form (Ewers and Fisher, 1991). The rate of laminar flow through a pipe is proportional (other things being equal) to the square of its cross-sectional area and to the pressure difference between its ends, and inversely proportional to its length and to the viscosity of the liquid. The number of vessels per square centimetre cross-section of functional xylem is approximately inversely proportional to the average cross-sections of each vessel. Higher flow rates are therefore expected to result from having a small number of vessels of large cross section rather than a large number of vessels of small cross-section. However, the flow of water in xylem is the result of the stress induced by the column of water by evaporation in the spongy mesophyll of the leaf (water is not pumped up from below), and the flow of water depends upon unbroken columns of water from the root tips to the mesophyll via the xylem. Too great a stress can cause the column to break, creating a gas-filled gap called an embolism. The greater the diameter of the xylem vessels, the lower the stress at which this breakdown occurs. The engineering and physiological solution to this dilemma is a compromise between conflicting requirements, and an optimal solution has to be found by evolution through natural selection. There has been a strong evolutionary incentive for plants to achieve optimal solutions, which has resulted in the development of xylem vessels, and the centrifugal growth (secondary xylem) that is characteristic of all woody vascular plants except palms and cycads. This has been a very important factor in the evolution of large plants since the Carboniferous age. In modern times, the problem is particularly severe for lianas. Lianas apparently show strong selection for conductive efficiency and exhibit higher specific conductivities and higher sap flux compared with angiosperm and gymnosperm trees (Ewers, 1985; Chen et al., 2017; Ichihashi et al., 2017). In contrast to their increased water-conducting efficiency, lianas were found to be less tolerant to drought-induced cavitation (embolism) than co-occurring trees (Van der Sande et al., 2013; Chen et al., 2017). However, the combination of wide and efficient vessels together with embolism-resistant narrow vessels and tracheids provides a small but sufficient transport capacity during dry periods. This pronounced dimorphism of vessels is highly characteristic of lianas and can be interpreted as a strategy to combine efficient water conduction with sufficient precaution against embolism (Carlquist, 1985). Trees, in contrast, rarely achieve both high efficiency and high safety (Gleason et al., 2016).
Hedera helix has solved these problems by a judicious combination of morphological and physiological adaptations, and is an excellent model species in which to study the solutions that evolution has devised to solve this dilemma. Because of the weight of water and cumulative resistance to flow in a column of xylem (Tyree and Ewers, 1991), the optimal morphology and physiology of xylem in the main stem at the crown is different from that at ground level. Branches cantilever out from the stem, and therefore require a different anatomy. I suspect that, in the branches, the number of leaves supplied, and therefore the transpiration load, per unit cross-section of functional xylem is smaller than the equivalent ratio for the main stem. However, I have not been able to find published work on this aspect. Be that as it may, the differences in requirement have led to a dimorphism between the ‘juvenile’ phase of the plant at ground level, and the ‘mature’ phase in the crown, especially in the structure of the stems.
An important physiological adaptation is that the stomata of Helix close at a lower water potential than those of nearby trees. This protects against the risk of embolism (Ganthaler et al. (2019) and Leuzinger et al., (2011), but temporarily shuts down photosynthesis. High water potentials are most likely to be encountered on hot, dry summer days, at a time of year when most plants in temperate zones expect to achieve their highest rates of photosynthesis, so this adaptation entails a considerable sacrifice of photosynthetic output. I think this may be an important reason why Hedera helix is evergreen, because it is less likely to be obliged to shut down photosynthesis in winter.
The dimorphism between the juvenile and mature phases is brought about by gibberellins: other plant hormones seem not to be involved (Rogler and Hackett (1975). Once induced in the crown, it is normally irreversible: cuttings of H. helix taken from mature-phase branches develop into small, non-climbing shrubs or trees. (K. Thompson, pers. comm. to Diedrich and Swearingen (2000). However, the change can be reversed by gibberellic acid.
ADVENTITIOUS ROOTS of the main ascending stem
The main stem of climbing Hedera attaches to its substrate by means of a large number of small adventitious roots… They arise in the phloem of the main stem in response to contact with the substrate, and attach to the substrate by means of adhesive nanoparticles (60-85 nm diameter) which appear to be composed of glycoproteins (dead ivy detaches from its substrate after a time, presumably when this glue is destroyed by microflora). This allows Hedera to attach strongly to a wide range of substrates, provided that they are not too smooth, such as glass or aluminium (Melzer et al., 2009 and 2010). The arrangement of these adventitious roots, in pairs, is thought to provide optimal resistance to shearing and peeling forces that might detach Hedera’s stem from its substrate. If Hedera is climbing a tree with exfoliating bark, and an area of bark to which Hedera is attached becomes detached from its tree, then Hedera’s attachment becomes useless. Authors recognise that having sacrificial exfoliating bark is an adaptation to repel epiphytes and parasites (Melzer et al. (2012), but I have not been able to find an article that explores how Hedera overcomes this problem, as it seems to do.
Tree ring analyses revealed increasing growth rates during the first 25 years of the life span of the studied ivy plants), when the liana was progressively extending its upper canopy. Mean annual xylem growth during the most recent 5 years settled at 0.82±0.06 mm, comparable to values reported for ivy in alluvial environments by Castagneri et al. (2013) (1.05±0.54 mm) and Heuzé et al. (2009) (0.77–1.89 mm).
Within-plant hydraulic variations were pronounced between branches and the main stem and with increasing branch insertion height, but small between the juvenile and adult life phases.
by Roger West with photographs by Chris Jeffree
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