Ranunculus peltatus Schrank, Pond Water-crowfoot

Account Summary

Growth form and preferred habitats

Like several other species and forms within the Ranunculus subgenus Batrachium, in permanently wet habitats R. peltatus is entirely perennial provided it is not overgrown or shaded by larger or more robust species. When growing in water, the plant normally produces both floating lobed laminar leaves and finely-dissected submerged capillary ones. However, if the site R. peltatus occupies dries out in summer, the species behaves as an annual and becomes both dwarf and prostrate, or even somewhat tufted in appearance (Cook 1966a; Webster 1988). As far as we know the annual form of the plant has never been reported as such in Fermanagh, but at least in grazed ground in and around turloughs (ie vanishing lakes rarely occurring in limestone districts), it could very well be the normal mode of the species.

In Ireland, R. peltatus is most typically found in more or less unshaded stretches of slow to moderately-flowing streams and rivers in calcareous districts. However, it can also occur in similar flow rates in much more acidic situations over peat or clay substrata and it appears to occupy a rather broad range of habitats in terms of nutrient status.

It should be noted that despite the species epithet 'peltatus', none of the organs of the plant, including the floating leaves, are ever peltate in form. The botanical name is misleading and a definite misnomer, arising as an 18th century error made by Prof. Carl Linnaeus himself! Further remarks on this topic are made below in the section 'Names'.

Occurrence in Fermanagh

In Fermanagh, this perennial or sometimes annual aquatic has been recorded in a total of just 14 lowland tetrads, 2.7% of those in the VC. It grows in shallow water in calcareous rivers, streams, lakes and ponds. The main site in Fermanagh for R. peltatus appears to be the Colebrooke River, where both R. peltatus and R. penicillatus subsp. penicillatus (Stream Water-crowfoot) occur. R. peltatus is also found at the Green Lough turloughs (ie limestone lakes or ponds that drain vertically – often referred to as 'vanishing lakes', since they dry out quite regularly), and it is found in an additional five or six rather scattered locations in the VC.

Although locally R. circinatus (Fan-leaved Water-crowfoot) appears in four more tetrads than R. peltatus, the latter is probably the commonest aquatic species of Ranunculus subgenus Batrachium in the VC, provided that is, R. hederaceus (Ivy-leaved Crowfoot) is discounted, since at least in our survey the latter is more often a terrestrial rather than an aquatic species.

Variation and identification

According to An Irish Flora (1977), specimens of R. peltatus with semi-circular to circular floating leaves produced in spring and summer (ie in addition to the constantly present finely divided, submerged, capillary leaves) and with large flowers (at least 18 mm across), should be reasonably easy to recognise as this species. In reality, however, there is considerable environmentally and seasonally induced variation in R. peltatus and it overlaps with R. penicillatus subsp. penicillatus (Stream Water-crowfoot), both in terms of morphological characteristics and ecology. Thus these two forms, in particular, are very readily confused (Cook 1966a; Holmes 1979; Webster 1988, 1991; Rich & Jermy 1998, p. 60).

Prior to Cook's 1966 monograph on Batrachian Ranunculi and for many years afterwards, due in part to the inadequacies of the Irish field Flora in widespread use here (An Irish Flora 1977), the few records we have of this closely related group of species were regularly lumped as R. aquatilis agg. or R. aquatilis s.l. (Common Water-crowfoot). Three truly aquatic members of the subgenus Batrachium, namely R. peltatus, R. aquatilis and R. trichophyllus (Thread-leaved Water-crowfoot), all exhibit very similar patterns of variation, both genetic and in response to the environment (ie genotypic and phenotypic variation), which Cook (1966a) believes indicates their close parental affinity. When these three species are in a submerged or terrestrial, vegetative, wholly divided-leaf state, they are in fact morphologically indistinguishable. More work is definitely required to sort out the differences and clarify the distribution of the water-crowfoot species in Fermanagh and, since the problem is universal, the same is true everywhere in the British Isles and, indeed, throughout Europe (Preston & Croft 1997; New Atlas).

Nutrient ecology and tolerance of disturbance

In the Sheffield area of England, Grime et al. (1988) believed R. peltatus was biased towards mildly acidic (often peaty) habitats, and that it was more or less absent from calcareous waters, where it was replaced by R. trichophyllus. This is definitely NOT the case in Fermanagh. Like R. hederaceus and R. lingua (Greater Spearwort), R. peltatus also appears to favour lowland aquatic sites which are subject to a degree of disturbance, appearing for instance in drainage ditches, quarry pools, and streams and ponds in areas that are regularly grazed and trampled by cattle or other stock animals, or in waters polluted from time-to-time with manure and agricultural chemical run-off (Cook 1966a; Hong 1991). One is here reminded of Grigson's aside in his essay on alien plants entitled 'The Wandering Flower', where he writes, "You may not be able to say so to a botanist, but nature interfered with by man is rather more fascinating than nature left to itself." (Grigson 1952, p. 114).

Cook (1966a) regarded R. peltatus and its two very closely relatives R. aquatilis and R. trichophyllus, as being characteristic pioneer plants of freshly dug or recently cleaned pools and ditches. All three rarely occur in water deeper than 100 cm and they do not tolerate deep shade or very swiftly flowing water. Cook also regarded R. peltatus and R. aquatilis as being confined to eutrophic waters although in Fermanagh we regard R. peltatus as a plant of more or less mesotrophic conditions. Cook never found these two species sharing a habitat, while both of them may frequently be intermingled with R. trichophyllus (Cook 1966a, p. 134).

In addition to the above habitats, in low-lying coastal areas of Britain and Ireland R. peltatus also occurs in dune-slacks and in lagoons. In S England, it frequents the uppermost reaches of calcareous rivers, which like the Irish turloughs, tend to dry out in summer (C.D. Preston, in: Preston et al. 2002).

The effect of drought on wetland plants

While numerous studies exist on the tolerance of wetland plants to flooding, in comparison very little research has looked at the effects of drought periods on such species. A worthwhile exception is Volder et al. (1997), who carried out a pot experiment on R. peltatus from wetlands in the French Camargue. This study found that the stage of development at which drought onset occurred had a major impact on the growth form and seed output of the species. R. peltatus plants subjected to the most severe experimental drought treatment (ie air drying of the soil with no added water), changed their vegetative growth form and survived for up to two weeks. Despite the very severe drought conditions, their flower, fruit and seed production, while very low, was not zero. The ability of R. peltatus to survive and produce viable seed under a wide range of hydrological conditions ensures a regular input to the seed bank. This fact helps explain the success of the species in temporary marshes and other wetland habitats, where exposure to drought stress is a regular or even an irregular occurrence (Volder et al. 1997).

Flowering and pollination

How the breeding system of R. peltatus operates is a matter of minor dispute. Cook (1966a) describes pretty well all of the Batrachian Ranunculi as being protogynous (ie female organs develop first) and self-pollinated, with a tendency to cleistogamy (a term literally meaning 'closed marriage', ie flowers self-pollinate in the bud). As he describes it, there is a variable length of time during which un-pollinated stigmas are exposed and outbreeding might occur. In R. peltatus, Cook reckons this duration can be up to 48 hours, but it is dependent upon the prevailing weather conditions being warm and bright. The flowers produce nectar and do have a scent similar to Cratageus monogyna (Hawthorn) blossom (Cook 1966a, p. 183).

In a study of Batrachian Ranunculi flowering behaviour in S Sweden, Hong (1991) agreed with Cook's findings for R. aquatilis and R. trichophyllus, but found no open anthers in the flower buds of R. peltatus. When the flowers opened, the stamens spread horizontally and gave good separation (c 5 mm) from the stigmas. In other Batrachian species Hong examined, the stigmas withered within a few days, but in isolated flowers of R. peltatus they remained fresh, swollen and looked receptive for periods up to 15 days. Beetles were observed visiting the flowers and Hong considered that they were the real agents of pollination in this species (Hong (1991) cited in Proctor et al. 1996). Other observations led Hong to suggest there might be a measure of self-incompatibility, such as definitely exists in R. acris (Meadow Buttercup). Jonsell et al. (2001) go further in this matter, stating that in Scandinavia R. peltatus is an obligate outbreeder, but somehow it retains a low degree of self-compatibility.

Overall reproductive strategy

As is often the case, nothing is known regarding the relative significance or frequency of successful establishment of sexual versus vegetative reproduction in R. peltatus, but as with R. penicillatus, a successful vegetative process is certainly easier to imagine happening. Certainly the species forms large stands and these probably consist of one or just a few clones, propagating simply by fragmentation (Jonsell et al. 2001).

British and Irish occurrence

The New Atlas map shows R. peltatus having a widely scattered, decidedly patchy, although quite frequent British and Irish distribution (Preston et al. 2002). However, the data must be qualified with an appreciation of the fact that all the Batrachian Ranunculi species are difficult to recognise and distinguish, and errors occur at an unknown frequency. We must therefore approach all maps of their distribution with caution and do not regard the presented pattern as totally reliable.

Having said this, the New Atlas hexad map for Ireland shows more frequent recording of R. peltatus in NE Ireland than elsewhere, a pattern that probably reflects recorder effort. In Great Britain, the species presence appears to thin out and become increasingly confined to coastal situations in both SW England and to the north of the Glasgow-Edinburgh conurbations in Scotland. In Scotland, at least, this probably reflects increasing altitude, but it will also mirror the number of available expert field recorders capable of identifying the species of this difficult plant group.

European occurrence

The maps in Cook (1966a, Fig. 15), Jalas & Suominen (1989, Map 1888) and Jonsell et al. (2001, p. 261), show R. peltatus widespread throughout Europe from the Mediterranean to the far N of Scandinavia, but with an overall distinct western concentration of records.

World occurrence

Beyond Europe, R. peltatus occurs only in the coastal area of N Africa and in parts of Asia Minor adjacent to SE Europe, but a closely related form called R. sphaerospermus, occurs in SW Asia (Cook 1966a; Hultén & Fries 1986, Map 872; Preston & Croft 1997; Jonsell et al. 2001).

Names

The Latin specific epithet 'peltatus' is derived from the Greek 'pelta' meaning 'a shield'. In plants, the technical term 'peltate' most often refers to the attachment of the stalk or petiole directly to the under-surface of the leaf blade, rather than to the blade margin in the normal manner (Gilbert-Carter 1964; Gledhill 1985). As mentioned above, the name 'peltatus' was given to this species in error by Carl Linnaeus in 1751, since no member of the Batrachian Ranunculi ever develops peltate leaves or other organs (Cook 1966a, p. 123). However, since the species name complies with the international rules of botanical nomenclature and is correct in every other respect, it has priority and must be retained. In this instance, the name is best considered merely a label, rather than as is more usually the case, descriptive, reflecting a specific character of the plant.

Threats

Shading and overgrowth by more vigorous plants encouraged by eutrophication.

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