Ranunculus ficaria L., Lesser Celandine

Account Summary

Growth form and habitat preferences

While R. ficaria is very commonly recorded in over half the area of Fermanagh (see below), we still regard this familiar yellow-flowered, patch- or carpet-forming tuberous perennial as under-recorded to some extent. The plant is a geophyte, having its resting or over-wintering buds protected by shallow burial in the soil. The reason we consider R. ficaria may be under-recorded is because the plant is pre-vernal, completing its annual growth and reproductive cycle early in the year and then quickly dying down and becoming inconspicuous, usually by the middle of June or earlier in some sites (Grime et al. 1988). Dead leaves, however, can still be found and the species recognised in suitable habitats later in the season and there is no month in which we do not have at least a few records of Lesser Celandine in the flora database. The short growing season of the shoots characterises R. ficaria as an ephemeroid perennial, the shoots disappearing before the summer (Rogers 1982).

R. ficaria occurs in a wide range of habitats and light environments, from deciduous mixed woodland where it is most frequent (except in those on the most acid soils), to hedges, roadside banks, verges, stream-sides and damp pastures. It is also found in gardens and can become a persistent weed of old lawns and flower beds (Taylor & Markham 1978; Sinker et al. 1985). Rapid growth very early in the season (ie its prevernal habit) enables the species to avoid the shade and competition of other plants, particularly in deciduous woodland but also to some extent in its other habitats.

Lesser Celandine tolerates a wide range of soils in these islands, both chemical and physical and, indeed, it appears to show a wider amplitude in this respect in Britain and Ireland when compared with its behaviour in Continental Europe (Hill et al. 1999). Measurements of soil reaction by Taylor & Markham (1978) showed the pH of the substrates occupied by the species ranged from 4.4-6.9 in the British Isles and that it frequently occurs on seasonally wet or flooded situations. It does not grow on very acidic mull soil (pH 3.9 or lower) and is absent from both permanently waterlogged or regularly droughted dry soils.

The small tubers or bulbils and their roots are buried only shallowly in the top 5-10 cm of soil and litter so that R. ficaria is confined to soils that are moist in spring, but which may possibly dry out later in the summer months. This dehydration does not matter as far as the plant is concerned, since by the time moisture stress occurs in the substrate, it is likely to be in its resting phase and therefore completely resistant to drought.

Competitive ability

Given adequate spring moisture, Lesser Celandine is most frequent where soil and vegetation is average with respect to fertility, productivity, level of disturbance and extent of bare uncolonised soil surface. Although it is a poor competitor in well illuminated sites, R. ficaria plants grow and perform better in ecologically open conditions in terms of producing a larger than average shoot, bigger individual tubers in the soil and more of the plants produce flowers and fruit (Taylor & Markham 1978).

The flora associated with Lesser Celandine typically has a relatively low level of diversity and this is particularly so in damp mixed woodland. Here, under the shade and protection of tree trunks, bare branches and the occasional evergreen or wintergreen leaf and frond, as the population of R. ficaria increases and begins to form extensive patches, species diversity declines further as it begins to oust other herbs. Thus under damp, shaded growing conditions, Lesser Celandine can sometimes form a more or less dominant, single-species carpet of ground vegetation (Taylor & Markham 1978; Grime et al. 1988).

Tolerance of disturbance

Plants can survive grazing and trampling by farm stock including horses, and mowing on grassy paths, roadside verges, and to a lesser extent in lawns. Due to the early season nature of its active period, annual growth and reproduction of R. ficaria plants is often near completion before these sorts of ecological disturbance reach any great level of intensity (Grime et al. 1988).

Phenology

Growth of R. ficaria is initiated by low temperatures in the autumn, and buds on the buried tubers begin to develop in December. The long-stalked, deeply notched, cordate foliage leaves of the basal rosette are very variable in shape. They are also shiny, hairless and often blotched with radiating pale or dark markings. The leaves begin to appear above ground in mid-January and they are well deployed by early February. Seedlings of the diploid, subsp. ficaria, require at least two year’s growth before they are capable of flowering (Marsden-Jones 1935).

Flowering

On established plants the flowering stem develops in February: it varies from 3-20 cm long and is spreading and weakly decumbent, branched and typically it bears one or two opposite leaves, similar to the basal ones but generally more distinctly lobed. Solitary flowers are borne at the end of each branch and flowering reaches a peak in late March and continues into April. The flowers are 20-30 mm in diameter and appear in a range of yellow shades (Grime et al. 1988, 2007).

Compared with other buttercup species, individual flowers of R. ficaria vary much more widely in the number of their floral parts, including the sexual organs. Early in the season many more stamens and carpels are produced per flower than in the ones opening later (Lee 1902). Thus the flower is atypical for the genus Ranunculus, usually having just three sepals and anything between seven and 13 petals instead of the more normal five of each organ characteristic of the genus (Sell 1994). Together with the apparently opposite and unequal stem leaves, these floral part numbers have led some taxonomists to separate the species off into its own genus and name it Ficaria verna Huds. (Hutchinson 1972).

There is not any concrete evidence suggesting that more open habitats favour flower and seed production by individual plants, which is the particular strategy of subsp. ficaria, the diploid (but see below). Higher light levels do allow more diploid plants to flower, however, and as would be expected, very heavy shade prevents any flowers being initiated. The apparent preference of R. ficaria s.l. for damp habitats may well be related to the conditions required to maximise the formation and survival of the perennating root tubers which are common to all ploidy levels (Nicholson 1983).

Pollination

However we decide to name this species, the flower structure is completely open and available to all insect visitors and the presence of nectar attracts short-tongued bees, honey bees, small beetles and flies whose visits achieve pollination. In wet cold weather the flowers remain closed and no insects visit. The amount of sun per day is a significant factor affecting early season insect-pollinated flowers, since whenever the sun becomes obscured there is always a sudden drop in temperature and a consequent great falling off in the number of insect flower visits (Marsden-Jones 1935). In the absence of visitors, self-pollination can occur, but the number of seed set is low in such circumstances, some flowers proving completely self-sterile and others producing very much reduced numbers of viable seed (Marsden-Jones 1935).

Fruit

The fruit of Lesser Celandine is a cluster of single-seeded achenes that are shed from the receptacle of the flower by early June. The number of achenes produced per flower varies with the fertility and suitability of habitat growing conditions (see below). Between 15 and 35 achenes may form on the receptacle of the flower, although often many of these potential fruit fail to develop or simply abort (Taylor & Markham 1978; Jonsell et al. 2001). Shortly after the achene clusters on the stem tips ripen and break up, the aerial parts of the plant wilt and quickly decay.

Genetic variation − two subspecies

It has been realised since the mid-1930s that Ranunculus ficaria occurs in the British Isles as two races with different chromosome numbers, a diploid, subsp. ficaria L. (2n=2x=16) and an almost sexually sterile tetraploid, subsp. bulbilifer Lambinon (2n=4x=32) (Marsden-Jones 1935). However, there is a problem distinguishing the diploid and tetraploid forms in the field, since whenever the species is flowering and is at its most conspicuous, the main character (the only field one), which distinguishes the two subspecies − the aerial bulbils of subsp. bulbilifer − have not yet begun to develop (Gill et al. 1972). In the 2nd edition of Stace's New Flora of the British Isles, additional vegetative distinguishing characters of a critical nature are provided, but this involves microscopic examination to count the chloroplasts in leaf pore guard cells (stomatal guard cells).

Vegetative reproduction − two forms of bulbil

The diploid form of R. ficaria is fully fertile and generally sets seed, but unlike other situations involving polyploidy where complete fertility attaches to even numbered chromosome sets, for some reason in this species the tetraploid form has a very low level of sexual fertility. By way of reproductive compensation, as it were, and undoubtedly encouraging or enabling their survival, the tetraploid plants possess aerial bulbils in addition to the subterranean vegetative bulbils that are typical of the species. Aerial bulbils are not produced at all on the diploid plant (Nicholson 1983). The small white aerial tubers are produced in the angle between a stem leaf and the shoot bearing it and should not be confused with the white or buff, fig- or spindle-shaped tubers which are produced just below ground level around the basal leaf rosette and which sometimes become exposed if the soil around the plant becomes disturbed. Basal tubers occur in all forms of Lesser Celandine plant irrespective of chromosome number and are both the perennating organ and a primary means of population increase and dispersal in many situations. The aerial bulbils are derived from swollen axillary adventitious roots exactly as the basal tubers are, so the structures are distinguishable really only by their position on the plant (Gill et al. 1972). It is clear from where they are produced on the plant that both types of bulbil originate as modified buds.

Reproductive strategy

The reproductive strategy demonstrated by R. ficaria s.l. is very common in, or even characteristic of, Arctic environments where the frequency of polyploid species is much higher than is found in the floras of temperate regions. It is estimated that 51% of the British and Irish flora is of polyploid origin, whereas the comparable figure for Iceland is 72% (Löve & Löve 1974). In Arctic regions, the season of growth in some years is sometimes too short for plants to complete their sexual processes and form viable seed. In such circumstances, natural selection favours the survival of species with a fall-back option of well-developed vegetative reproduction. The growing season of vernal herbs is also brief in woodland and other deciduous seasonally shaded habitats, growth in pre-vernal and vernal herbs being confined to the light phase of the year, from autumn leaf fall to the expansion of the new leaf canopy the following spring. During at least part of the long dormant or overwinter period, low temperatures obviously restricts or prevents active plant processes (Taylor & Markham 1978). When temperatures again increase, the active growth phase of the pre-vernal plant is so early in the year, changeable weather conditions make activities like pollination uncertain and some processes, including seed set, may actually fail. Therefore species such as R. ficaria, Hyacinthoides non-scripta (Bluebell) and, in Great Britain (but not in Ireland), Mercurialis perennis (Dog's Mercury), rely very heavily on vegetative reproduction for their increase and survival.

R. ficaria tetraploids have a greatly reduced seed output, but as discussed above, they have two methods of vegetative propagation − aerial bulbils and basal tubers. In seasonally shaded, relatively undisturbed habitats where opportunity for seed dispersal is poor, this may well prove the better arrangement in terms of both plant survival and spread into new ground.

Fermanagh occurrence

In Fermanagh, R. ficaria s.l. is commonly recorded in 297 tetrads, 56.3% of those in the VC. As previously mentioned, it is probably under-recorded to an unknown extent since its growth is early season, pre-vernal and the plant dies down completely by June. The habit of distinguishing these two subspecies only gained ground slowly in Britain and Ireland and it is regretted that they have not been distinguished in any of the Fermanagh fieldwork to date.

British and Irish occurrence of the two subspecies

The distribution of subsp. ficaria is described by Stace as occurring, "throughout the British Isles", and for subsp. bulbilifer, "almost throughout, but apparently absent from Shetland and the Channel Isles" (Stace 1997). The British and Irish occurrence contrasts strongly with that in Nordic regions, where the tetraploid is the common and widespread subspecies and the diploid is very rare and confined to Denmark and the southern tip of Norway (Jonsell et al. 2001).

In the 6th edition of An Irish Flora (the book relied on for most of the current Fermanagh survey by Irish recorders), Webb (1977) indicated that a variant (ie subsp. bulbilifer) existed and was, "occasionally found, mainly in the E, and usually as a garden weed". This impression is partially confirmed by the Flora of County Dublin, where subsp. ficaria was described as occurring, "very common in woods and hedges, and very rare in gardens". In contrast, the same work described subsp. bulbilifer as being, "very common in gardens, disturbed roadsides, and also in woods and hedges" (Doogue et al. 1998).

Triploid forms and aerial bulbils

Where the diploid and tetraploid overlap,

triploids result from their crossing (2n=3x=24) (Gill et al. 1972). Triploid plants may flower but they are totally sterile. They sometimes produce aerial bulbils, but these are smaller and fewer than those of the subsp. bulbilifer tetraploid plants.

Comparison of subspecies with respect to light and shade

While R. ficaria s.l. is most commonly found in summer shaded habitats such as woods, scrub and hedgerows, as with the Wood Anemone (Anemone nemorosa) and the Bluebell (Hyacinthoides non-scripta), in the west of Britain and Ireland it also grows in open situations in meadows and rough pastures. Clapham et al. (1962) suggested that the diploid, subsp. ficaria, is the more commonly found form in the British Isles, especially in sunny sites and that the tetraploid, subsp. bulbilifer, is more restricted to shady places. In Yorkshire, Nicholson (1983) found that while the diploid occurred under the full range of light levels, the tetraploid was associated with moderate to heavy shade and was absent from high ground, including the Wolds and the cliffs at Flamborough Head, and it was also absent from low sandhills in the coastal region. This is a rather surprising claim since these areas appear to represent more physically demanding environments, and by analogy with the pattern of arctic plants where higher degrees of ploidy and associated vegetative apomixis occur in the more severe growing conditions. One would therefore expect the tetraploid form with its additional means of vegetative reproduction to be favoured, rather than excluded from the more demanding environments.

Field observations and experimental measurements both suggest that subsp. ficaria is the more light tolerant form of the two, rather than suggesting subsp. bulbilifer is more shade tolerant (Nicholson 1983). The observation that R. ficaria is able to persist in more open meadow conditions in the N & W of the British Isles is thus more probably due to the prevalent moist, cool conditions in these regions, rather than suggesting any requirement the subspecies has for shade.

There is not any concrete evidence suggesting that more open habitats favour seed production by individual plants, which is the particular reproductive strategy of the diploid subsp. ficaria (but see below). However, higher light levels do allow more diploid plants to flower and very heavy shade will prevent flowers from being initiated. The apparent habitat preference of R. ficaria s.l. for damp ground may well be related to the conditions required to maximise the formation and survival of the perennating root tubers, which are common to all ploidy levels of the plant (Nicholson 1983).

Comparative seed production

In measurements of sexual reproductive performance, the mean seed output computed for sun and shade diploid plants was very similar (75 and 71 per plant), giving an overall mean of 73 ± 5.8. (Marsden-Jones 1935). Flowers on tetraploid plants in open conditions tended to be produced earlier than those on the diploids, but even if they become pollinated, only a low proportion of their carpels set viable seed. Marsden-Jones (1935) collected a total of just 23 apparently viable achenes from ten tetraploid plants, a figure representing around 2% of the ovules produced by these plants. A large proportion of the pollen of tetraploids is also non-viable, unlike that of the diploid form (Taylor & Markham 1978).

Seed dispersal

After fertilization in late May and early June, the head of achenes ripens and dries somewhat so that it breaks up at the slightest touch. By this time, or soon after, the other aerial parts of the plant have already begun to wilt and decay. There does not appear to be any special dispersal mechanism for the seed and presumably it does not travel more than a few centimetres. The fruiting stems do elongate, however, and become more horizontal as they age which helps dispersal, and some seed might be carried further by rain wash, especially over bare compacted soil or along tracks and paths.

Seed dormancy and germination

Seeds of R. ficaria require a period of after-ripening as the embryo is only partly differentiated when they are shed. A delay of four to six months and a period of low temperature is necessary to break dormancy. Germination begins in the following spring and continues until early summer. Germination rates are poor for both subspecies, ranging from 15-46% for the diploid and from 7-29% for the much rarer seeds of the tetraploid (Taylor & Markham 1978).

Seedlings have only one cotyledon

Seedlings of R. ficaria, like that of at least some populations of Conopodium majus (Pignut) are highly unusual for a dicotyledonous plant group, in having only one embryo leaf or cotyledon and not two (Taylor & Markham 1978). This fact has been known since the 1850s and was the subject of much speculation among botanists for many years. The seedling's typically bilobed blade and the venation in the single cotyledon suggested to some botanists that it might have arisen by the fusion of two cotyledons. However, in a beautiful piece of anatomical and embryological investigation carried out at Kew Gardens, Metcalfe (1936) clearly and carefully argued and conclusively proved that the cotyledon is in fact a single foliar organ brought about by the suppression of one of the cotyledons during embryo development, and not the product of two embryo leaves becoming fused together. Part of the evidence for this is the rare occurrence of R. ficaria seedlings with two bilobed cotyledons, and trilobed single cotyledons have also been observed. Metcalfe's discovery involved recognising the existence of a rudimentary second cotyledon which normally fails to develop and thus seedlings only produce a solitary embryo leaf or cotyledon.

A comparison might be made here with the genus Cyclamen (Primulaceae) members of which also have only one cotyledon, the other aborting during early development. Another case in point is the genus Peperomia (Piperaceae), where some species are heterocotyledous: one cotyledon being a green, aerial, assimilating organ, while the other is retained inside the seed when it germinates and serves primarily for the absorption of food reserves from the seed (Metcalfe 1936). Other examples of this type of odd phenomenon include some species in the genus Claytonia including C. virginica (Stebbins 1974; Mabberley 1997). [N.B. This very interesting paper by Metcalfe has been incorrectly attributed to Marsden-Jones and given the wrong date by two recent authors, one of whom also got the volume number wrong! It is correctly quoted, attributed and cited here.]

Tuber and bulbil dispersal

All clones of R. ficaria irrespective of chromosome number possess root tubers (around twelve per cluster) and the tetraploid subsp. bulbilifer also has its aerial axillary bulbils. Fragmentation of the basal tuber cluster by soil disturbance is a very efficient means of vegetative propagation. In addition, the tetraploid produces up to 24 bulbils per plant that separate off and drop to the ground as the shoot bearing them dies down. A heavy shower of rain will sometimes wash these aerial bulbils away from the parent plant, heaping them together on the sides of run-off channels when the rain ceases. In such places, the quantity of bulbils aggregated is often so noticeable that the idea arose that they had fallen from heaven with the rain, and the myth developed of 'potato rain', or a 'rain of wheat' (Grieve 1931; Taylor & Markham 1978).

Tubers and bulbils provide the most rapid means of increase and spread for the species (and subspecies), as they are readily detached and may be spread by disturbance of any kind, but especially by digging, ploughing or mowing. Bulbils being considerably larger than seed, can regenerate the plant quicker and some may even flower in their first season of growth (Marsden-Jones 1935).

Origin of polyploidy in R. ficaria: In view of the quite different ecological behaviour and reproductive strategies of the diploid and tetraploid forms, it appears very likely that subsp. bulbilifer is an ancient autotetraploid, ie it is the result of mutant chromosome doubling, followed by a long period of divergence of the two ploidy levels (Nicholson 1983). Genetic isolation barriers have not yet been established, however, since in Britain and Ireland (though apparently not in Nordic areas (Jonsell et al. 2001)), triploids are formed by interbreeding where the two subspecies overlap (Gill et al. 1972). Thus Lesser Celandine is a very good example of species evolution in action, and indeed in other parts of Europe several additional subspecies are recognised (Sell 1994; Jonsell et al. 2001).

Toxicity

R. ficaria plants contain low levels of the toxin irritant protoanemonin, an unstable compound derived from the glycoside ranunculin. The concentration of this toxin increases during growth and it is at its highest during the flowering process. Unlike many other buttercups it has not been known to poison grazing stock animals (Cooper & Johnson 1998). Despite the presence of the toxin, young leaves of R. ficaria often have holes eaten in them in early February, and we have seen entire blades removed at this time of year, presumably eaten by slugs before the protoanemonin levels become a functional deterrent.

European occurrence

External to the British Isles, the diploid R. ficaria subsp. ficaria is restricted to W Europe, north to SW Norway and Denmark, where it is extremely rare, and south to The Iberian peninsula and the W Mediterranean region (Jalas & Suominen 1989, Map 1836). The bulbil-bearing tetraploid, R. ficaria subsp. bulbilifer is also confined to Europe, but it is very much more widespread in C and S parts of the continent than the diploid form (Jalas & Suominen 1989, Map 1835; Taylor & Markham 1978).

Fossil record

There is no fossil record of R. ficaria s.l. as the pollen and possible macrofossils are indistinguishable from other Ranunculus species (Taylor & Markham 1978).

Names

The specific epithet 'ficaria' is derived from the Latin name 'Ficus', the fig, and it means 'small fig'. This is a reference to the supposed fig-like shape of the root tubers (Gledhill 1985).

Two of the numerous English common names are 'Figwort' and 'Pilewort', 'fig' and 'pile' being alternative names for haemorrhoids, which the root tubers resembled. The similarity in appearance led herbalists by the 'Doctrine of Signs' to believe that a plant organ that looked like a haemorrhoid could be used to cure the complaint (Grieve 1931; Grigson 1987). Certainly the herb contains an astringent, but while Vickery (1995) includes a folklore recipe for a skin cleanser, it should be left well alone since the sap can cause blisters on or in the body (Grieve 1931).

Other English common names such as 'Brighteye', 'Butter and Cheese', 'Golden Stars' and 'Goldy Knog', refer to the shiny, yellow petal colour. Grigson (1987) also suggests the root tubers were regarded as reminiscent of a cow's udders, and hence the milk and butter references in some common names. An early botanical name for the species was Chelidonium minus, which translates as 'Lesser Celandine'. This erroneously linked the plant to the unrelated Chelidonium majus (Greater Celandine). The name 'Chelidonium' was derived from the Greek 'chelidon', a swallow (the bird), supposedly because of coincidence between the time of the plant flowering and the arrival of the migrant bird.

Threats

None.

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