Persicaria amphibia (L. ) Gray ( Polygonum amphibium L.)
2001; Wiley; Volume: 89; Issue: 3 Linguagem: Inglês
10.1046/j.1365-2745.2001.00571.x
ISSN1365-2745
Autores Tópico(s)Plant Pathogens and Fungal Diseases
ResumoA versatile rhizomatous perennial, one of the rather few truly amphibious plants (Rodwell 1995), with aquatic and terrestrial forms so different that the uninitiated may think that they are different species (Lousley & Kent 1981). Aquatic form. Pale, branching rhizomes < 0.2 m deep in underwater substrate (Best & Dassen 1987), and < 2 m under water (Spence in Veg. Scot.), carry perennating buds that produce pliable, sparsely branching stems with vestigial leaves and rings of adventitious roots; internodes 5–40 cm (Arber 1920). Stems branch < 10 cm underwater, and bear alternate floating leaves: blades pliant, waxy, smooth, 4–10 cm long, 2–4 cm wide, lanceolate/narrow-ovate, margin entire or minutely toothed; petioles 2–8 cm, flexible. Ochreae entire, membranous, semitranslucent, secreting mucilage around axillary bud. In early summer, shoots surface with 1–3 aerial leaves and inflorescences. Peduncle glabrous or sparsely hairy ± stalked glands; otherwise aquatic form entirely glabrous. Terrestrial form.± glabrous, glandular-hairy, finely to coarsely pubescent or appressed-hispid. Rhizomes < 50 cm deep produce procumbent/erect stems, 0.4–1 (–2) m, branching below, little-branching above; internodes 1–4 cm, nodes (+ root-precursors) swollen. Ochreae entire, green, hairs (if present) may exceed margin. Petioles ± rigid, 0.5–1.5 cm, blades 4–14 cm long, 1–3 cm wide, more opaque, coarse and inflexible than in aquatic form, lanceolate, margin fine-toothed, immature/early leaves often with dark chevron-shaped blotches. Transitional form at water-margins.± glabrous, intermediate petiole-length and leaf-shape. Inflorescence similar in all growth-forms: two principal sexual types on male-fertile and male-sterile clones; probably gynodioecious, see Section VIII(A). Peduncle 2–7 cm, terminal, rarely axillary, unbranched or with 1–3 branches, blunt compressed panicle/complex raceme c. 1.5–6.0 cm long, 1 cm wide, of c. 30–150 flowers on 2–5 mm pedicels in groups of 1–3 within bracts (ochreolae) 1–2 mm. Perianth five-lobed (tepals), lobes fused half-way below apex, white, pink or deep rose-red, eglandular. Male-fertile flowers: bell-shaped, 4–5 × 4 mm, five exserted stamens, 4–6.5 mm long, inserted at perianth base alternately with five orange nectaries; anthers 1–2 mm. Ovary single, 1 mm, unilocular, bifacial, central at perianth base; style single, 3–4 mm, usually not exserted, branched in upper 1/2–3/4, 2 (3) capitate stigmas. Male-sterile flowers differ in: perianth 3–4 × 2–3 mm, tubular, style 4–5.5 mm, exserted, stamens 0.5–1 mm, empty, atrophic. Achene bifacial, biconvex, 2–2.5 mm, brown, air-dry weight 3.8 ± 0.4 mg (n = 40; J.W. Partridge, unpublished data). The plant's mutability was the subject of classical studies on the relationship of growth-forms to environmental and genetic influences, reviewed by Mitchell (1968). Systematists, confused by phenotypic modification superimposed on genotypic diversity, described as separate taxa variations now regarded as mere growth-forms. Britton (1933) listed five such 'varieties'; current British usage abandons them (Kent 1992; Stace 1997). The alternative nomenclature of the plant, Polygonum amphibium L., has been included in the Section Persicaria (Miller) DC. of Polygonum L. s.l. (Fl. Europ. 1, 2nd edn). The genus Persicaria Miller was considered sufficiently distinct to merit generic rank (Haraldson 1978; Ronse Decraene & Akeroyd 1988; Ekman & Knutsson 1994), and British practice now is to use Persicaria amphibia (L.) Gray (Kent 1992; Stace 1997). White-flowered plants with cordate leaves from North-West China and far-Eastern Russia have been recognized as P. amphibium L. var. amurense Korsh. (Fl. USSR V). There are two currently recognized North American endemic varieties: P. amphibium L. var. stipulaceum Coleman, and var. emersum Michx; the former has flared stipular appendages (Mitchell 1968), but the distinctions are subtle. In summary, P. amphibia is a rhizomatous perennial with aquatic and terrestrial growth-forms: the former has underwater stems and glabrous leaves, the latter erect or decumbent stems and pubescent leaves. The inflorescence is a dense, complex raceme, rarely produced by the terrestrial form; clones are either male-fertile or male-sterile. Widely distributed and with rapid vegetative spread, it is native in aquatic habitats and adventive in disturbed habitats; it is particularly adapted to changing water-levels. Persicaria amphibia is widespread and native throughout the British Isles (Fig. 1), usually below 200 m, though at 572 m in the Lake District (Stokoe 1983) and 290 m in Wales (Seddon 1972). Lack of suitable habitats rather than altitude may explain its apparent absence in upland areas in Britain. The vegetative terrestrial form may be overlooked or mistaken for Persicaria maculosa (an annual) and the aquatic for Potamogeton natans (differing in parallel leaf-veins). Grime et al. (1988) considered that the plant was decreasing, yet Rich & Woodruff (1996) reported an increase in records between 1930–60 and 1987–88. The records in 2000 (Fig. 1) have increased since the 1962 distribution map of Perring & Walters (Atl. Br. Fl.). The distribution of Persicaria amphibia in the British Isles. Each dot represents at least one record in a 10-km square of the National Grid. (○) Pre-1950, (●) 1950 onwards. Mapped by Jane M. Croft (Centre for Ecology and Hydrology, Monks Wood), using Alan Morton's DMAP programme, mainly from records collected by members of the Botanical Society of the British Isles. In Europe, P. amphibia is widespread (Fig. 2; Fl. Europ., 1, 2nd edn). It reaches 1200 m in the Tyrol (Kerner von Marilaun 1902). The distribution of Persicaria amphibia in Europe. Reproduced from Jalas & Suominen (Atl. Fl. Eur. 1976) by permission of The Committee of the Flora for Europe and Societas Biologica Fennica Vanamo. World-wide it has a northern circumpolar boreo-temperate distribution, category 56 of Preston & Hill (1997), and is widely naturalized elsewhere, including South America, South Africa and Mexico. In the Atlas mountains of North Africa, P. amphibia grows at 2100 m (Maire 1961), in Bhutan at 3950 m (Grierson & Long 1983) and in the Kashmir Himalaya at 2000–2800 m (Munshi & Javeid 1986). Although P. amphibia as a British native plant may be restricted to wetland (or former wetland) sites (Grime et al. 1988), human activity has dispersed it to road verges, railway banks, allotments, grassland, arable fields, spoil heaps and waste tips. Maritime habitats include shingle beaches (Tansley, Br. Isl.), a calcareous maritime loch (Spence in Veg. Scot.), brackish estuaries (Beeftink 1975) and sand-dune slacks (Lousley & Kent 1981). Persicaria amphibia grows best with full sunlight or only light shade, in relatively nutrient-rich water or well-irrigated deep soil. Grime et al. (1988) found that it was restricted to flat sites or slopes less than 20°, but was in all their 'bare soil' classes and in all but the driest of their six hydrology classes. It grows in still water or in streams and rivers with a slow to moderate flow, and can tolerate seasonal spates (Haslam et al. 1975) but not vigorous wave action in large lakes (Raspopov et al. 1978). Inundation transforms it into the aquatic form, allowing an almost unrivalled, although transient, supremacy: after East Anglian floods, this plant and Alisma plantago-lanceolata covered c. 6500 hectares (Compton 1916). The floating form, adopted at water depths of 30–200 cm, mean 100 cm (Spence in Veg. Scot.), is tolerant of turbidity caused by sediment or algae (Preston & Croft 1997), mesotrophic or eutrophic conditions and brackish saline water, but not high salinity. Regular and high rainfalls are beneficial; in prolonged summer drought it becomes xeromorphic and fails to flower (see Section V, Part C). A persistently windy situation can have the same effect in spite of adequate soil hydration (J.W. Partridge, unpublished data). Persicaria amphibia grows in sand, clays, loam, peat, shingle, manure, underwater silt and black mud. Soil pH is between 3.0 and 8.0, and is usually > 4.5 (Central Northern England; Grime et al. 1988). It tends to be absent from 'very eutrophic or limy waters' (Rodwell 1995). In Scotland, the plant grew in the following water values: Loch Clunie, pH 7.2, 16 p.p.m. CaCO3, 'low–moderate alkalinity'; Loch Lindores, pH 7.9, 52 p.p.m. CaCO3, 'moderate alkalinity'; Loch an Aigeil, pH 8.58, 146 p.p.m. CaCO3,'rich alkalinity'. At Loch Lindores, the substrate was 'black reducing mud (warp-anmoor)' (Spence in Veg. Scot.). In the English Lake District, it grew 1.5 m deep in a substratum of pH 5.44, organic content 8.66% dry weight, total nitrogen content 0.27% dry weight and which was strongly positive for sulphides (Misra 1938). In an East Anglian fen it grew in raw peat silt, weight loss by ignition (humus) 70–84% (Lambert 1945). Seddon (1972) found it in 19/70 Welsh lakes in mesotrophic and eutrophic conditions, minimum water conductivity 100 µS cm−1 and hardness ratio 2.5; maximum conductivity 235 µS cm−1 and total dissolved solids 155 mg L−1 and CaCO3 + MgCO3 116 mg L−1. A British river habitat survey recorded the 'amphibious vegetation types'P. amphibia and Agrostis stolonifera in 18% of sites, and they were extensive in 0.4%, on river-slopes of c. 2–12 m km−1 and widths of c. 0.7–3 m, on all substrates and in all flow-types (Dawson et al. 1999). It was present in 26/50 Danish lakes (Iversen cited in Hutchinson 1993), classified as: 'strongly acid', pH always < 5.3, 2/10 lakes; 'variable', pH 4.4–6.9, 2/7 lakes; 'variable, not always acid', 7/9 lakes; 'circumneutral', 4/5 lakes; and 'alkaline', pH 7–9, 11/19 lakes. The water clarity in Finnish lakes where P. amphibia grew was Secchi disc transparency 2.5–7.0 m, mean 3.9 (Maristo cited in Hutchinson 1993). In a German survey, it was found in 10/59 meadow and riverbank sites where the topsoil available mineral nitrogen levels were between 5.50 and 11.84 mg N per 100 cm2 soil (F.H. Meyer cited in Ellenberg 1988). Table 1 lists pH, moisture and organic matter content for soil samples collected from various British locations of the plant. In disturbed terrestrial habitats, P. amphibia occurs with a heterogeneous vegetation mixture, such as the 181 species in a Leicestershire refuse tip (Primavesi & Evans 1988; habitat 104). In a study of a wide range of habitats in Central Northern England, Grime et al. (1988) found that the associated floristic diversity of the plant was 'low to intermediate', with c. 7–13 species m−2, more diverse in terrestrial than aquatic habitats: it shared the latter with Alisma plantago-aquatica: 98%, Eleocharis palustris: 97%, Glyceria maxima: 95%, Hydrocotyle vulgaris: 94% and Typha latifolia: 92%. In open water it may accompany Nymphaea alba, Nuphar lutea and Potamogeton natans (Haslam 1978), the Nymphaeion (Hutchinson 1993), which also included the Myriophyllo-nupharetum. Spence (in Veg. Scot.) described, with plant lists, the 'P. amphibium sociation' in Scottish lochs, and the plant occurred in 70% of Glyceria maxima–Iris pseudacorus sociations of loch-margins; these two sociations did not seem to be subject to any but the slowest of succession patterns. The sociations are also widely distributed in British lowlands (Haslam 1978). In East Anglia, Lambert (1945) recorded P. amphibia moderately constantly in three communities in the lakes (Broads) and fenland meadows: the Glycerietum, a dense semifloating mat dominated by Glyceria maxima; the Phragmitetum, an emergent community in peat and silt with Phragmites australis; and the Juncetum, fenland meadow with Juncus subnodulosus (species lists provided). Dawson & Szoszkiewicz (1999) classified the Polygonetum amphibiae Association in the Order Potamogetonalia of Class 2 Potamogetonetea in a survey of vegetation associations in British rivers. In the river Wye, bordering Wales and England, Edwards & Brooker (1984) found it growing, almost continuously, 100–250 km from the source, and described the river hydrology and its macrophytes and bryophytes. Habitat lists that include P. amphibia are also provided in Floras of British Vice-counties (e.g. Cadbury et al. 1971; Primavesi & Evans 1988). The British Plant Community survey recorded P. amphibia (Rodwell 1991, 1995, 2000) in 29/257 communities of the National Vegetation Classification: 11/24 Aquatic communities, 13/28 Swamp communities, 3/42 Open habitat communities and 2/38 Mire and heath communities. Only in two of these communities, both Aquatic, did the plant occur in a sufficient number of sample habitats to be considered 'constant' (Category IV or V, 61–100% samples). In the remainder it was either 'scarce' (Category I, 1–20% samples) or 'occasional' (Category II, 21–40%), and its abundance within individual samples was usually in the authors' lowest categories: 'sparse' or 'plentiful'. Only the two aquatic communities where it occurred in 61–100% samples ('constant', Categories IV or V) will be mentioned (Rodwell 1995). Community A4, Hydrocharis morsus-ranae–Stratiotes aloides community was almost exclusive to East Anglian Broadland, in mesotrophic base-rich still water; P. amphibia occurred in 61–80% samples (Category IV) but was 'sparse' within them. This community had been previously described by Wheeler & Giller (1982) as one of three communities in undrained fenland in Broadland, and they gave details of water and sediment chemistry. Persicaria amphibia was found in the majority of stands in these communities, but with a cover of only 0–5%. Community A10, Polygonum amphibium community, was recognized by Rodwell (1995) in 37 samples as a floating-leaved association widespread throughout Britain in standing water. The plant was present in 81–100% samples (Category V) and its abundance varied from 'sparse' to 'dominant'. On average, it was accompanied by three other species from among the following: Nuphar lutea, Potamogeton gramineus, P. natans, P. obtusifolius, P. perfoliatus, Lemna minor, Glyceria fluitans, Callitriche stagnalis, Equisetum fluviatile, Elodea canadensis, Littorella uniflora, Nymphaea alba and Juncus filiformis. The final volume of this extensive survey (Rodwell 2000) provided a summary of the plant communities described, and their equivalent designations and synonyms. There are currently 196 vegetation samples containing P. amphibia listed in the National Vegetation Classification database (Dr A.J.C. Malloch, personal communication). Hutchinson (1993) also usefully summarized the aquatic vegetation classifications and plant lists that include P. amphibia, from other European countries. Some representative examples follow. In Czechoslovakia, Ellenberg (1988) described the plant in a flood-plain sward community, the Rorippo-Agrostietum, where in the wetter depressions it may attain dominance over Rorippa sylvestris, Agrostis stolonifera, Potentilla anserina and Potentilla reptans; a similar community is found in some seasonally flooded British water-meadows and river-sides. He also described it in the Phragmitetea in wooded flood-plains dominated by Salix alba, the Salici-Populetum, a community unfamiliar in Britain; in general the plant rarely occurs in British woodland because of its poor shade-tolerance. Ellenberg (1988) also records P. amphibia in the Myriophyllo-Nupharetum of the Potametea, and in the Spirodelo-Salvinietum and the Lemno-Azolletum of the Lemnetea; similar aquatic communities occur in Britain but with some variation in their constituent species. In an ecological study of Dutch ditches, De Lange (1972) found P. amphibia in 5/7 vegetation subclasses and 13/15 vegetation units, with positive associations with Elodea nuttallii, Butomus umbellatus, Potamogeton lucens and P. compressus; an exhaustive analysis of European ditch vegetation classification was appended. In sand-dune slacks of several islands of the Wadden Sea, P. amphibia is recorded as an important associate species in, for example, the Scirpetum fluitantis and Sparganietum minimi of the Class Littorelletea and in the Menyanthes trifoliata association of the Alliance Caricion nigrae (Petersen 2000). In many aquatic habitats, this plant is an effective competitor because it can adapt to fluctuating water-levels. In the warm effluents of power stations, it grows luxuriantly with other macrophytes (Efimova & Nikanorov 1977). Its leafy tangles can choke lakes, reservoirs and slow-moving rivers; attempts to clear them can disseminate the plant further. Similarly, ploughing and cultivation break up the rhizomes, which can spread laterally more than 50 mm a day (Sculthorpe 1967). Stem fragments form adventitious roots quickly if wet: within 2 days in the laboratory (Hodgson cited in Grime et al. 1988). Persicaria amphibia produces secondary growth in response to grazing and mowing, which also further distribute stem fragments and prevent succession to scrub (where P. amphibia cannot compete well because of its poor tolerance of shade, though it can scramble as high as 2 m in hedgerows alongside wet places). In a eutrophic Welsh lake, P. amphibia was displaced by Nymphoidespeltata, a species non-native to Wales (Jones & Benson-Evans 1974). Persicaria amphibia competed poorly with P. lapathifolia (annual, seed-producing) and P. hydropiperoides (Michx) Small (North American seed-producing annual or perennial) in experimental conditions; Carter & Grace (1990) concluded that its strategy was to sacrifice short-term competitive success for long-term survival. The plant has long been regarded as a 'mischievous weed' (Greville cited in Syme 1868), a 'pessimum vitium' (Leers cited in Stanford 1925b) and 'difficult to control and persisting' (Hanf, Weeds). Glyphosate applied at 1.5–2.0 kg ha−1 achieved long-term elimination (Weed Handb., 8th edn). As a water-weed, it is resistant to Dalapon and Terbutryn, and is only temporarily suppressed by Dichlorbenil, Chlorthiamid, Diquat (Weed Handb.) or Paraquat (Way et al. 1971). For potential sources of biological control see Section IX. The plant is relatively tolerant of pollution and increasing eutrophication: it is often one of the last aquatic macrophytes to survive (Best 1982; Srivastava & Gupta 1995), although eventually it is eliminated (Lachavanne 1979). Särkkäet al. (1978) detected pesticides and mercury in aquatic P. amphibia at a site where mercurials had been discharged 4 years earlier. Lead levels in P. amphibia from a pond deliberately seeded with lead-shot 3 years earlier suggested that the plant actively absorbed lead, but did not store it in the rhizomes (Behan et al. 1979). Yalynskaya & Lopotun (1993) measured the levels of 15 heavy metals and microelements in P. amphibia. In an Indian river heavily polluted with industrial and sewage discharges (Srivastava & Gupta 1995), this was the only dominant aquatic macrophyte growing at all study sites – as an aquatic in the least polluted site, as a luxuriant marginal at a sewage outlet, and as a sparse marginal at the site with maximal industrial pollution – reflecting the levels of heavy metals in soil and river water, and nutrient, oxygen and pH levels in the water. Periphyton growth on the aquatic plant in USA marshland included genera of green algae (e.g. Cladophora, Chlorella, Spirogyra) and diatoms (e.g. Fragilaria) (Cronk & Mitsch 1994). In a Dutch lake (Dvorak & Best 1982), P. amphibia sheltered 700–1050 individual macro-invertebrates, per 10 g ash-free dry weight of plant material: Mollusca, Oligochaeta and Insecta (Ephemeroptera, Trichoptera and Diptera). In a Russian lake, the benthos and water surrounding the plant contained three categories of organisms; microbenthos: c. 256 120 organisms m−2 (of surface area of benthos); macrobenthos: c. 46 220 organisms m−2 (of surface area of benthos); zooplankton: c. 110 100 organisms m−3 (of lake water) (Kurashov et al. 1996). Its shade only weakly inhibited the development of associated green algae, diatoms and cyanobacteria in a Russian lake (Ali & Khromov 1992). Ogan (1982) showed the consistent presence of nitrogen-fixing bacteria on the surface of aquatic roots of P. amphibia, and also found seasonal changes in the numbers of these bacteria associated with excised roots of the plant from a Scottish loch. Persicaria amphibia is stand-forming in water-margins and in deeper water. It is possible that these stands may be single vegetative clones, extending several kilometres along riverbanks and canals, and over many hectares of lakes, water-meadows and flooded fens (Compton 1916; Raspopov 1979; J.W. Partridge, unpublished data). In terrestrial vegetation, the plant is often not stand-forming, but is intermingled with other colonists. As an inhabitant of shallow water-margins, one of the most productive habitats on earth (Best 1982), P. amphibia is an important primary producer. As with other macrophytes, it removes and releases nutrients, and is a source of detritus and a substrate for micro- and macrofauna and microflora. The productivity and relative growth rate of floating-leaved stands in a Dutch lake with a plant density of 40 m−2 were measured (Best & Dassen 1987). Standing crop, the weight of plant material expressed as grams ash-free dry weight (AFDW), in June was 160 g AFDW m−2 when leaves were developing and becoming maximal, 305 g AFDW m−2 (September), diminishing to 167 g AFDW m−2 (November). The overwinter standing crop was 66–74 g AFDW m−2. The leaf area index (the total surface area of leaves per unit ground area) followed a similar rise and decline. The photosynthetic area per unit of standing crop reached a maximum of 0.007 m2 g−1 AFDW. Tree-shading allowed only 33% of photosynthetically active radiation into the colony; the net efficiency of solar energy conversion was 0.68%. The average relative growth rate for the active period 10 June–3 September was 0.008 day−1. In a previous study, increased tree-shading may have caused the fall from the maximum total biomass production rate of 0.81–0.63 g m−2 day−1 in the following year (Best 1982). In the Rybinsk reservoir, USSR, biomass production at shore level was 2.93 g m−2 wet weight (dry weight 0.43 g m−2); at 0.9 m water depth, 0.46 g m−2 wet weight (dry weight 0.12 g m−2) (Hrbácek 1985). Carbohydrates and nitrogenous compounds in P. amphibia reach their peak before flowering, when harvesting for animal fodder would be most appropriate (Yakubovskii & Merezhiko 1975). In Britain, an aquatic plant extended 13 m in a season (Arber 1920). In fertile terrestrial situations, underground lateral spread is rapid (Sculthorpe 1967). In Leicester botanical garden, clones, transplanted from 10-cm2 pots, covered surface areas from 0.7 to 2.25 m2 in the first season, with rhizomes continuing underground up to 15 cm beyond the margins of above-ground growth. Turesson (1961) propagated 20 clones in aquatic and terrestrial conditions. All 20 clones flowered when aquatic, and produced significantly more flower-spikes per plant than in terrestrial conditions where only 8/20 clones flowered. In water, 9/20 clones changed from the terrestrial form into an aquatic form with lateral growth, while 11/20, though producing leaves of a typical 'aquatic' form, had ascending stems. Similarly, on land, some clones adopted an erect habit, others were procumbent, and this predisposition was independent of the aquatic growth-form of the clone. Clones varied in their readiness to transform from the terrestrial to the aquatic form. Mitchell (1968, 1976) had similar results with North American material. The plant is very sensitive to frost, often the first to die back in autumn in Britain, and early growth may be killed by late spring frosts. If the water does not freeze solid and there is air beneath the ice, aquatic colonies may retain some stems above the sediment, unlike the terrestrial form that almost invariably dies back to below ground level in winter, although in Central England terrestrial plants growing near an industrial warm-water outlet developed woody above-ground stocks that survived a mild winter (J.W. Partridge, unpublished data). Drought and drying winds induce a hispid xeromorphic form (Britton 1933) with prostrate stems, short internodes and narrow, undulate leaves; growth slows and flowers never form, or fail to develop, and seeds are not set. Conversely, heavy rainfall, high humidity and water-logging lead to rapid growth, the development of aquatic or transitional-aquatic growth-forms and increased flowering and seed-set. Seeds survive overwinter only in damp, cool conditions (see Section VIII). Morphology, including stomata, indumentum type and the histology of leaf blades, petioles and stems, depends on environmental as well as genetic influences; differences in these structures have been used taxonomically (Mitchell 1968; Haraldson 1978; Osetrova 1986). Fundamental morphological changes enable the plant to respond to rapid water-level fluctuations. In underwater parts, epidermal cells elongate and often contain chlorophyll, palisade tissue is reduced or absent, vascular bundles are less developed (particularly xylem), stomata are not formed and gas-filled spaces (aerenchyma) appear in leaves and stems; these changes are reversed as new organs and tissues develop when the plant becomes terrestrial (Hutchinson 1993). Stomata may be anomocytic, anisocytic or paracytic (Haraldson 1978; Munshi & Javeid 1986), and occupy c. 2.5% leaf area. Mitchell (1971) measured stomatal index and frequency in glasshouse-cultivated plants of the North American varieties of the plant: var. stipulaceum Coleman, stomatal index: 0.97, stomatal frequency: upper surface 140–180 mm−2, lower surface 150–190 mm−2; intermediate varieties, stomatal index: 0.58, stomatal frequency: upper surface 30–74 mm−2, lower surface 90–114 mm−2; var. emersum Michx, stomatal index: 0.60, stomatal frequency: upper surface 130–260 mm−2, lower surface 220–310 mm−2. The aquatic growth-form was not studied; stomata are absent on the lower surface of this form. Gaberscik & Martincic (1992) provide a statistical analysis with a dendrogram of the changes in leaf morphology and stomatal distribution in samples of plants from four aquatic and terrestrial locations. Stomatal frequency increases and trichome distribution and frequency decrease with increasing humidity at the leaf surface. Haraldson (1978) recorded the presence and distribution of various trichome types. Mitchell (1968) described a 'branched' trichome, specific to some North American plants and the vegetative 'flares' on juvenile stipules of an endemic variety. Stamen insertion and number (Ronse Decraene & Akeroyd 1988), and pollen grain morphology (Hedberg 1946) have also been used taxonomically in Polygonaceae, but are relevant only to male-fertile clones (approximately 50% of P. amphibia in Britain). Pollen grain morphology is described from European (Hedberg 1946), American (Stanford 1925b) and Indian material (Munshi & Javeid 1986). The rhizomes, diameter 0.3–1.0 cm, branch irregularly and produce roots at nodes, with terminal and lateral buds, which are embryonic shoots with several short internodes enclosed in corky scales and mucilage. In summer the rhizomes become packed with starch grains, distorting the conducting system. Rhizomes become woody with up to three annual rings (Mitchell 1968). Harley & Harley (1987) recorded no mycorrhizas. However, very sparse endomycorrhizal colonization, with hyphae only, was seen in 1% of roots collected from North American plants growing in dry zones of wetlands, but none was seen in roots from plants in a water depth of at least 10 cm (Rickerl et al. 1994). In the Raunkiaer classification, P. amphibia may be a floating hydrophyte, an emergent hydrophyte, a geophyte or a protohemicryptophyte, illustrating the difficulties of applying this system to some aquatic plants (Sculthorpe 1967). Vegetative reproduction is very effective by the spread of stems and rhizomes, either directly by rapid growth in soil or water, or when disseminated by human activity, wave action, water-currents and perhaps by birds and large mammals during grazing. A single 2-cm section containing a node can start a new colony (Grime et al. 1988). In contrast, sexual reproduction is uncertain and infrequent (see Section VIII). Other polyploid members of the Polygonaceae also depend largely or exclusively on vegetative reproduction, for example, Fallopia japonica var. japonica, which has spread in Europe as an invasive weed stems from a single male-sterile clone (Hollingsworth & Bailey 2000). Kerner von Marilaun (1902) described a Tyrolean colony observed annually for 28 years never to set seed, which propagated itself 'with rare luxuriance by offshoots'. There are several vegetative colonies that have never flowered or set seed for at least 12 years in Central England (J.W. Partridge, unpublished data). In cultivation, clones were maintained by Turesson (1961) for 22 years or longer. From seed it took 4 years for a plant to flower in Leicester botanical garden, and 3 years from a 2-cm cutting. The only published count for British material is 2n = 96 (Al-Bermani et al. 1993), but there is an unpublished value of 2n = 88 for an Irish plant (Gornall & Bailey 1993). Values from elsewhere are 66, 88 and ± 96 (Ch. Eur. Pl., Chr. Atl.; Turesson 1961). Chromosomes of P. amphibia are small and hard to count; my own unpublished results on British material suggest a diploid count of 2n = 96. Mitchell (1968), referring to his unpublished haploid result of n = 50 ± 2 on North American material, speculates that the plant was of allopolyploid origin and had arisen only once from tetraploid ancestors. Nuclear DNA (2C) amount is 2.0 pg (Grime et al. 1988). Concentrations of chlorophyll, ash, carbon, nitrogen and six minerals were measured at intervals during the growing season of the plant in a Scottish l
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