Glechoma hederaceaL. (Nepeta glechomaBenth.,N. hederacea(L.) Trev.)

Abstract

A polycarpic, aromatic, often purplish, softly hairy or nearly glabrous perennial herb. Flowering shoots 10–30 (–60) cm long, square in section and ascending or erect. Non-flowering stems also square in section, creeping and rooting at many, but not all, nodes. Leaf laminae 4–35 (–80) × 6–40 (–80) mm, reniform to ovate–cordate, obtuse or subacute and coarsely crenate. Petioles highly variable in length from 10 mm in grassland to over 200 mm in woodland with dense canopy cover and dense ground vegetation. Bracteoles 1–1.5 mm, shortly hispid or pubescent. Flowers zygomorphic and hypogynous, borne in few (2–5)-flowered secund axillary whorls, the bracts not differing from the foliage leaves. Calyx tubular or tubular-campanulate, 15-veined, not gibbous at base, and somewhat two-lipped. Upper lip of the corolla flat with three triangular acuminate teeth that are 0.2–0.3 times as long as the tube. Lower lip with two teeth. Corolla tube narrowly obconical, straight and hairy within at the base of lower lip. Corolla (6–) 15–22 (–25) mm, usually blue–violet with purple spots on the lower lip; rarely pink or white. Stamens four, two long and two short, borne on corolla tube and lying below upper lip; anther cells at right angles to each other, each opening by a separate slit. Ovary four-celled and deeply four-lobed, each cell with one ovule. Style gynobasic, its branches equal. Hermaphrodite and male-sterile forms of flowers occur, the latter usually with smaller corollas than hermaphrodite flowers, and much-reduced, pollen-less stamens that are often included in the corolla tube. Clones usually bear either only hermaphrodite or only male-sterile flowers, but in rare cases both types are found on the same individual (Price 1991). Each flower produces a maximum of four obovoid, slightly angled, smooth nutlets with a mean air-dry mass of 0.69 mg (n = 100). Each is slightly angled owing to pressure exerted by the adjacent developing nutlets before dispersal, and each bears a grey protuberance which may be an elaiosome. The dimensions of the seed are 1.9 × 1.1 mm. Glechoma serbica Halácsy & Wettst., which has been recorded in former Jugoslavia, is probably an ecological modification of G. hederacea (Fl. Eur. 3). The southern counterpart of G. hederacea in Europe is ssp. hirsuta (Waldst. & Kit.) F. Hermann, sometimes referred to as G. hirsuta (Elias 1979, 1983). This has denser and longer hairs and longer calyx teeth than G. hederacea, and the corolla is pale blue. It has been recorded in Albania, Austria, Bulgaria, the Czech Republic, Greece, Hungary, Italy, former Jugoslavia, Poland, Rumania, central and south-west Russia, Sardinia and Sicily (Fl. Eur. 3). Intermediates, which are presumably hybrids between G. hederacea and G. hirsuta, sometimes occur (Fl. Eur. 3). In eastern Asia a more or less distinct, short-pubescent race occurs that is called ssp. grandis (A. Gray) Hara [also var. grandis A. Gray and var. longituba (Nakai) Kuprian] (Hultén 1971). In North America the variety micrantha Moricand [or var. parviflora (Benth.) House] is distinguished (Hultén 1971). Glechoma hederacea is native to the British Isles and common throughout except for western Ireland and northern Scotland, where it is rare (Fig. 1). It is not native in Orkney and Shetland, and is absent from the Outer Hebrides. It occurs almost throughout Europe, except in the Balearic Islands, Crete, the Faroes, Iceland, Spitsbergen and Turkey. It is a weed in Tadzhikistan (Hultén 1971). It is also found in western and northern Asia to Japan, in Hong Kong and in Tonkin (Hultén 1971). It has been introduced in New Zealand, where it is found in many southern localities in the North Island and in many localities throughout the South Island (Webb et al. 1988). Nutlets have not been found on any New Zealand material (Webb et al. 1988). It has also been introduced in America (Atl. Br. Fl.; Fl. Br. Isl.; Fl. Eur. 3; Hultén 1971; Fitter et al. 1985; Grime et al. 1988), where its range extends southwards through the United States to Georgia, Tennessee, Kansas and Colorado (Gill 1979). It is naturalized in Canada and ranges from Newfoundland (Lat. 47°N, Long. 53°W) to British Columbia (Lat. 54°N, Long. 123°W). The distribution of Glechoma hederacea in the British Isles. Each dot represents at least one record in a 10-km square of the National Grid. Mapped by Mrs J. M. Croft, Biological Records Centre, Institute of Terrestrial Ecology, mainly from records collected by members of the Botanical Society of the British Isles; (○) pre-1950; (•) 1950 onwards; (×) garden escapes or outcasts. The altitudinal range of G. hederacea has been given as from sea level in England, Scotland and Wales to 400 m in Weardale and 415 m at Burbage, Derbyshire, UK (Alt. range Br. Pl.). Other records are at altitudes of 410 m (Grime et al. 1988) and 295 m (Fl. Br. Isl.) in the British Isles, 350 m in Sweden (Hultén 1971), and to about 1600 m in the Alps (Hultén 1971). Glechoma hederacea is a species of temperate latitudes. Ellenberg Indicator Values (Ellenberg et al. 1991) are given as 6 and 3, and Landolt Indicator Values (Landolt 1977) as 4 and 3 for temperature and continentality, respectively, indicating that G. hederacea is primarily a species of the colline-submontane zones of temperate Europe, with an oceanic to suboceanic distribution. In the British Isles its northern limit follows the July isotherm of 12 °C (Atl. Br. Fl.). In Canada its northern limit follows the 12 °C July isotherm and the −12 °C January isotherm (Gill 1979). It occurs on sites with a wide range of slopes. In the Sheffield region it was slightly more common on south-facing slopes than on other aspects, but more abundant on north-facing slopes in unshaded situations (Grime et al. 1988). It is most abundant on sites with a moderate to high percentage of bare ground. Preston & Hill (1997) classify G. hederacea as Eurasian Boreo-temperate, but Circumpolar Boreo-temperate after wide naturalization [see also Hultén (1971); map 274 for G. hederacea and for sspp. hirsuta and grandis]. (B) SUBSTRATUM Glechoma hederacea occurs mainly on damp, heavy, fertile and calcareous soils. It is most frequent in the pH range from slightly acidic to slightly alkaline (pH 5.5–7.5), but can also be found on soils with a pH as low as 4.0 (Fl. Br. Isl.; Fitter (1978); Grime et al. 1988). Ellenberg Indicator Values for substrate (Ellenberg et al. 1991) agree with these substrate affinities, being 6 for humidity, × for soil acidity (i.e. no strong preference) and 7 for mineral nutrient availability. Landolt (1977), however, records that it does not tolerate strongly acidic soils. He also reports that it is intolerant of salinity and that it is mainly found on fine-textured soils including clays. Kreutzer & Seibert (1984) and Sinker et al. (1985) state that G. hederacea becomes abundant only when there is a good phosphate and nitrate supply and a moderately rich supply of cations including Ca2+ in the soil substrate. III. Communities Glechoma hederacea is typically a plant of shaded habitats including woodlands and hedgerows. It is frequent on shaded roadsides, in clear-felled and coppiced woodlands, scrub and derelict woodland, and in orchards. Although it is described as having only a limited capacity to persist under tall herbs (Grime et al. 1988), it is frequently a persistent associate of Urtica dioica and other species in both open habitats and at woodland edges (Sinker et al. 1985). Peterken (1981) regards G. hederacea as a characteristic species of secondary woodland (Hermy & Stieperaere 1981). It sometimes becomes locally abundant in damp woodlands (Fl. Br. Isl.). It is also found in grassland habitats growing in more or less full sunlight, beside rides and rabbit scrapes, on spoil heaps, wasteland, waysides, walls and at the edge of pastures, or encroaching and even dominating enriched areas at the margins of arable fields. Although it is generally absent from aquatic habitats (Cadbury et al. 1971; Hyde 1976; Fitter 1978; Grime et al. 1988), it is occasional in soligenous mires and on moist soil at the periphery of topogenous mires, on river banks and floodplains. Typical associates of G. hederacea include Ajuga reptans, Brachythecium rutabulum, Bryonia dioica, Galium aparine, Geranium robertianum, Geum urbanum, Hedera helix, Poa trivialis, Ranunculus ficaria, Ranunculus repens, Sambucus nigra, Stachys sylvatica, Stellaria holostea, Tamus communis and Urtica dioica (Pl. Comm. Scot.; Sinker et al. (1985); Grime et al. 1988). Species that occur in similar habitats to G. hederacea are listed by Grime et al. (1988) as Stachys sylvatica (habitat similarity 85%), Galium aparine (77%), Tamus communis (74%), Urtica dioica (70%) and Stellaria holostea (63%). In a study of the spatial variation of vegetation in an ancient oak–ash–hazel–maple woodland following coppice management (Ash & Barkham 1976), G. hederacea was listed, together with Arum maculatum, Carex sylvatica, Dactylorhiza fuchsii, Heracleum sphondylium, Listera ovata, Mercurialis perennis, Orchis mascula, Paris quadrifolia and Poa pratensis, as a species of shaded habitats rather than of persistent clearings. Glechoma hederacea is a component of several woodland communities described in the National Vegetation Classification (Rodwell 1991a). In most of these it has a recorded frequency class of I (i.e. a frequency of 1–20% in vegetation samples) but it reaches class II (21–40% frequency) in some subcommunities of W6 (Alnus glutinosa–Urtica dioica) and W8 (Fraxinus excelsior–Acer campestre–Mercurialis perennis) woodlands, and in some subcommunities of W24 (Rubus fruticosus agg.–Holcus lanatus) and W25 (Pteridium aquilinum–Rubus fruticosus agg.) underscrub communities. It is recorded as common (class III, 41–60% frequency) in the Primula vulgaris–Glechoma hederacea subcommunity (W8a) of W8 (Rodwell 1991a). Glechoma hederacea occurs in the Alnus glutinosa–Filipendula ulmaria subcommunity (W2a) of the Salix cinerea–Betula pubescens–Phragmites australis woodland community (W2; Rodwell 1991a) that often develops on abandoned mowing marshes. Like other herbs of low stature, plants of G. hederacea are scattered. It colonized the Wicken Fen woodlands described by Rodwell (1991a) as a variant of W2a approximately 50 years after mowing ceased, when cover of Phragmites australis and other tall herbs had declined, and Urtica dioica and Poa trivialis had increased. In this condition the field layer resembled that of Alnus glutinosa–Urtica dioica (W6) valley fen woodlands on moist eutrophic soils. In the diverse collection of subcommunities within W6, G. hederacea has a frequency of II both in the typical subcommunity (W6a), in which Urtica dioica is very abundant, and in the Sambucus nigra subcommunity (W6d), which is found on rather drier soils. Urtica dioica is less prominent in the latter subcommunity, and Rubus fruticosus agg., Lonicera periclymenum, Hedera helix and Dryopteris spp. often form thick underscrub and ground layers. The Betula pubescens subcommunity (W6e) also occurs on drier soils. Here, Urtica dioica is again less common, while Rubus fruticosus agg. and Lonicera periclymenum produce a bulky underscrub, and G. hederacea falls in frequency. In the Alnus glutinosa–Fraxinus excelsior–Lysimachia nemorum communities (W7) that occur on wetter mesotrophic mineral soils, G. hederacea occurs in both the Urtica dioica (W7a) and Deschampsia cespitosa (W7c) subcommunities, although its frequency and abundance are low in both. Together with Poa trivialis, Ajuga reptans and Primula vulgaris, G. hederacea is one of a group of field layer species that are consistently found in Fraxinus excelsior–Acer campestre–Mercurialis perennis (W8) woodlands on heavy, base-rich soils in south-east England, in woods where Quercus robur has a high frequency. These species are all less common in W8 woodlands in the north-east, except where the soil is locally moister. In the W8a Primula vulgaris–Glechoma hederacea subcommunity it becomes common and can occupy large areas in coppice or coppice-with-standards of Quercus robur. Rodwell (1991a) suggests that the evergreen habit of G. hederacea helps to prevent its competitive elimination by Mercurialis perennis, which is abundant in this subcommunity. Following coppicing, G. hederacea can rapidly re-invade trampled sites in this subcommunity by stolonifery, and perhaps by seed, since it often flowers abundantly following such management. Glechoma hederacea has a lower frequency in the Anemone nemorosa subcommunity (W8b) that occurs on wetter sites. It is also less abundant in the more open Deschampsia cespitosa subcommunity (W8c) that develops when woodland on moist sites is coppiced. Here it is initially outcompeted by Deschampsia cespitosa and by annual and biennial weeds that grow rapidly in the less shaded conditions following coppicing, but it rallies as the canopy grows back. Under the continuous tree and shrub canopy in which the Hedera helix subcommunity (W8d) develops, G. hederacea is restricted to those areas where H. helix is relatively sparse on the ground and not replaced by either Mercurialis perennis or Hyacinthoides nonscripta. In both the Geranium robertianum (W8e) and Teucrium scorodonia (W8g) subcommunities G. hederacea has a low frequency and abundance. Glechoma hederacea is recorded with low frequency and abundance in all subcommunities but W10e (the Acer pseudoplatanus–Oxalis acetosella subcommunity) of W10 (Quercus robur–Pteridium aquilinum–Rubus fruticosus woodlands). In W10a–d its abundance is limited by that of several other species, including Pteridium aquilinum, Rubus fruticosus agg., Lonicera periclymenum, Hedera helix (in W10c), Holcus mollis (in W10d), dryopteroid ferns and, in the vernal aspect, either Hyacinthoides nonscripta (W10a) or Anemone nemorosa (W10b). In such vegetation G. hederacea may become locally prominent on heavy, moist soil where the cover of Pteridium aquilinum is low. In the Mercurialis perennis (W12a) and Sanicula europaea (W12b) subcommunities of W12 (Fagus sylvatica–Mercurialis perennis woodlands), and in W13a (the Mercurialis perennis subcommunity of Taxus baccata woodlands), G. hederacea is a component of sparse and patchy field layers under deep canopy shade. Glechoma hederacea is a consistent component of all recognized subcommunities within W21 (Crataegus monogyna–Hedera helix seral scrub and hedge communities), being preferentially associated with the Mercurialis perennis subcommunity (W21b). It also ranges in frequency from scarce to occasional in subcommunities included in W24 (Rubus fruticosus–Holcus lanatus underscrub) and W25 (Pteridium aquilinum–Rubus fruticosus underscrub). In W25a (the Hyacinthoides nonscripta subcommunity) it can achieve ground cover values of between 50% and 75% (Rodwell 1991a). In continental Europe G. hederacea is a characteristic species of wood-margin, hedge bank and scrub communities. Oberdorfer (Pfl. Exk.) records it in Klasse Artemisietea vulgaris, Unterklass Galio-Urticenea in the Orders Glechometalia (7–66% frequency) and Artemisietalia (5–32% frequency), but it is virtually absent from Order Onopordetalia. In the Netherlands, Westhoff & den Held 1969) mention G. hederacea specifically as a characteristic species of the Galio-Alliarion Alliance (natural or semi-natural tall forb communities of hedge-sides and woodland edges on moist, fertilized soil), the Trifolion medii Alliance (scrub/wood-margin communities with, for example, Agrimonia eupatoria, Galium mollugo, Vicia sepium, Veronica chamaedrys and Dactylis glomerata), and the Order Alno-Padion (wet woodlands) of the Fagetalia sylvaticae. In mesotrophic grasslands, G. hederacea is a component of all subcommunities of MG1 (Arrhenatherum elatius grasslands) which, in the absence of mowing and grazing, are often transitional to mixed deciduous woodland (Rodwell 1992). Although it can reach ground cover values as high as 33% in the Urtica dioica subcommunity (MG1b), it falls in frequency class I in all but the Pastinaca sativa subcommunity (MG1d) that often develops on more calcareous and oligotrophic soils in the south-east when grazing is relaxed, or when arable cultivation on marginal chalkland has failed. In acid grasslands G. hederacea is an element of subcommunities U1b–U1d of the Festuca ovina–Agrostis capillaris–Rumex acetosella community (U1). All these communities have open swards with small tussocky grasses, abundant dicotyledonous herbs and substantial amounts of lichens and mosses. The open conditions, created by the death of plants owing to drought, and disturbance caused by grazing animals, probably present opportunities for colonization by G. hederacea through stoloniferous growth. Its cover reaches its highest values (up to 33% Rodwell 1992) in the typical subcommunity (U1b). Oberdorfer (Pfl. Exk.) records frequencies of 3–33% for G. hederacea in meadow communities dominated by Arrhenatherum elatius and Trisetum flavescens. It also occurs in the Lolio–Cynosuretum. Westhoff & den Held (1969) refer to G. hederacea as a characteristic species of Arrhenatherum elatioris in the Netherlands. Several ruderal communities contain G. hederacea at low frequency (Rodwell 1999). It occurs in the Arrhenatherum elatius–Rubus fruticosus agg. subcommunity (OV24b) of OV24. This Urtica dioica–Galium aparine community is species-poor, tall-herb vegetation on nutrient-rich, moist soil, and often associated with disturbance and abandoned farmland and dwellings. In OV25 (Urtica dioica–Cirsium arvense community), which has a more open and patchy cover of U. dioica than OV24, G. hederacea is a component of the Holcus lanatus–Poa annua (OV25a) and Lolium perenne–Papaver rhoeas (OV25c) subcommunities. On damp and fertile soil that has been disturbed or burnt, it occurs in the Urtica dioica–Cirsium arvense (OV27b) and Acer pseudoplatanus–Sambucus nigra (OV27d) subcommunities of the Epilobium (Chamerion) angustifolium community (OV27). Glechoma hederacea is also a minor component of the Alisma plantago-aquatica–Sparganium erectum subcommunity (S5b) of Glyceria maxima swamp vegetation (S5) and of the Epilobium hirsutum subcommunity (S26b) of Phragmites australis–Urtica dioica tall-herb fen communities (S26). Westhoff & den Held (1969) also refer to G. hederacea as a component of a community dominated by Epilobium hirsutum and Phalaris arundinacea in the detritus zone of lake margins. Finally, it is also recorded in the Equisetum variegatum subcommunity (SD15b) and the Holcus lanatus–Angelica sylvestris subcommunity (SD15d) of Salix repens–Calliergon cuspidatum dune-slack vegetation (SD15), and in the Urtica dioica–Arrhenatherum elatius subcommunity (SD18b) of Hippophaë rhamnoides scrub (SD18) (Rodwell 1991b). IV. Response to biotic factors The responses of G. hederacea to biotic and abiotic environmental factors can be fully understood only by considering the relationship between vascular anatomy and phyllotaxis in the family Lamiaceae, and the constraints which this places on resource movement within these species. Stems in the Lamiaceae are quadrangular in section. Although the xylem forms a continuous cylinder within the stems, the major vessels are concentrated near the angles of the stem. Phloem cells are confined to four discrete regions directly adjacent to the stem angles. Thus, four main vascular bundles run parallel with the four stem angles. At each node a single leaf emerges from each of two opposing faces of the stem. The faces from which leaves emerge rotate by 90° between adjacent nodes. This opposite decussate pattern of leaf emergence (Bell 1991) results in each stem or stolon having four orthostichies of leaves. Stolons of a higher branching order are in the same orthostichy as their subtending leaves. A simple set of rules (Price & Hutchings 1992a) controls the relationship between vasculature and the patterns of resource distribution between leaves and stolons that arise in different orthostichies. These determine the patterns of physiological integration and independence between leaves and ramets. They are as follows: (i) leaves in opposite orthostichies, and their axillary stolons, are largely physiologically isolated from each other; (ii) leaves in adjacent orthostichies, and their axillary stolons, exhibit partial physiological integration; (iii) leaves in the same orthostichy, and their axillary stolons, are potentially fully integrated. These rules apply within single stolons. However, as new stolon branches develop they acquire the status of integrated physiological units (IPUs; Watson & Casper 1984) that are largely physiologically independent of each other. This happens because an additional rule operates as follows: (iv) resource translocation in the stele of G. hederacea is predominantly acropetal. New axillary stolons develop from the leaf axils when nodes have been superseded by several younger nodes towards the stolon apex. These axillary stolons capture most of the translocates leaving the subtending leaf and resources from the roots supplying the same vascular bundle. Thus, few resources are distributed from older nodes to the apex of the original stolon. Instead, the apex is provided with resources by the leaves and roots at more distal nodes on the original stolon. At this stage, each of the older nodes on the original stolon has become a largely physiologically independent segment that supplies resources primarily to stolons growing from the axils of its leaves (Birch & Hutchings 1999). When the axillary stolon can support its own maintenance and growth, it retains most of the resources it produces and acquires, rather than exporting them basipetally to other stolons. Thus it achieves the status of an IPU (Price & Hutchings 1992a). Translocation of resources from older to younger ramets increases the probability of daughter ramet survival (Slade & Hutchings 1987a). Physiological integration appears to involve no costs, at least in biomass terms, to the source ramet, but confers growth benefits upon the whole clone (Slade & Hutchings 1987b). The vascular anatomy of the species constrains the possible pathways of resource movement within the clone. There is, for example, little evidence of reciprocal translocation within stolons (Stuefer & Hutchings 1994). Price et al. (1996) compared the growth and morphology of genetically identical G. hederacea clones grown in isolation and in competition with Lolium perenne. The grass was either clipped to maintain a height of 1 cm, or allowed to grow, finally reaching approximately 13 cm. In the short grass treatment G. hederacea mainly experienced competition for soil-based resources, whereas in the tall grass treatment there was competition for both soil-based resources and light. Although total biomass of G. hederacea was significantly reduced by the presence of competing Lolium perenne, it was not significantly affected by its height. Primary stolon growth was unaffected by competition, but the number and length of secondary stolons were significantly reduced in the short grass treatment, and there was a further significant reduction in competition with long grass. Internode and petiole lengths increased significantly in clones competing with tall grass, but were not significantly affected by short grass. The proportion of biomass allocated to stolons increased from 20% in clones grown without competition to 36–38% in clones subjected to short and tall grass competition (Hutchings & Price 1993). Allocation of biomass to petioles also increased, from 15% without competition to 21% with short grass competition and 35% in clones growing with tall grass. Both leaf mass and area were significantly lower when G. hederacea was grown with competition. Thus, clone morphology was considerably changed in response to competition, becoming more linear in the horizontal plane and more extensive in the vertical plane. This raised the horizontally orientated leaves to the top of the grass canopy. These responses would promote avoidance of competition in both the horizontal and vertical planes; energy invested in densely vegetated areas would be reduced, and occupation of less utilized sites enhanced. Reductions in photosynthetically active radiation (PhAR) and in the R/FR ratio incident on the plant, and raised humidity caused by the competitor, were all implicated in causing these morphological changes. Some treatments in this experiment examined the effect of habitat heterogeneity by growing different stolons of the same clone either with or without competition from either short or tall grass. Stolons treated in this way developed different morphologies, each statistically indistinguishable from the morphologies of stolons from clones growing under the corresponding homogeneous conditions. This is a consequence of the vascular structure of the clone, and the constraints imposed by this structure upon resource movement. Each stolon behaved as an IPU. This enabled each stolon to develop a morphology appropriate to its local conditions, rather than a morphological compromise for the variety of conditions experienced by the clone. Notably, the total yield of clones with one stolon competing with tall Lolium perenne and the other growing without competition significantly exceeded that of clones growing entirely without competition. While the shaded stolon produced a lower yield and fewer branches than equivalent stolons grown without competition, the growth of the unshaded stolon apparently benefited substantially from greater local humidity caused by the grass canopy shading the adjacent stolon. In the absence of accompanying shade this led to a proliferation of secondary and tertiary stolon production. The subspecies hirsuta, which is sometimes accorded species status as G. hirsuta, produces both orthotropic (vertically orientated) and plagiotropic (horizontally orientated) shoots. As in G. hederacea, shading reduces growth of plagiotropic shoots but has little effect on orthotropic shoots. Vertically orientated internodes of orthotropic shoots show a greater proportional increase in length in response to shade than horizontally orientated petioles, whereas the opposite is true for plagiotropic shoots, supporting the hypothesis that, within a species, responses to shade will depend on orientation of growth, and that vertically orientated structures should be more plastic (Huber & Hutchings 1997). The hypothesis proposes that because light gradients are more predictable in a vertical than horizontal direction, plasticity in the lengths of vertically orientated spacers will more often result in greater light acquisition. Internodes of the plagiotropic stolons of G. hederacea lengthen under shade and other adverse conditions, including low nutrient supply. This lengthening is accompanied by significant decreases in weight per unit length and stolon cross-sectional area, mainly achieved by reducing the volume of the parenchymatous tissue. The vascular system is considerably less plastic, thus conserving its capacity for resource transport (Price & Hutchings 1992a). In the field, G. hederacea has a limited capacity to persist under tall herbs or beneath a continuous tree canopy (see Section III above). Grime et al. (1988) describe it as a species that is most characteristic of lightly or patchily shaded habitats. It may become locally abundant and flower profusely after coppicing (Fl. Br. Isl.), but gradually decline in vigour, and even be excluded as the canopy regrows. A study by Ash & Barkham (1976) suggested that it has the capacity to persist for only 3 years after coppicing. Glechoma hederacea is toxic to many species, although many phytophagous invertebrates are known to feed on it. Price & Hutchings (1992b) analysed the effect on plant performance of simulated herbivory in the form of selective defoliation, to test the hypothesis that both the structural level at which G. hederacea responds to phytophagy, and the specific response elicited, will be determined by pattern of defoliation and by the extent of physiological integration between the defoliated ramets. Yield declined progressively in clones subjected to 0%, 50% and 100% defoliation. Leaves were removed in three patterns for the 50% defoliation treatments, and yield was significantly affected by removal pattern. Compared with undefoliated clones, similar reductions in yield and in several components of yield were caused when defoliation was achieved either by removing all leaves from one of two stolons and none from the other, or by consistently defoliating two of the four orthostichies on both stolons by removing one of the two leaves from every ramet. However, when consecutive pairs of ramets on each stolon were either defoliated or left intact, the effect on yield and its components was significantly smaller. Compared with undefoliated clones, this pattern of defoliation had little impact on growth at a primary structural level, although secondary growth was impaired. This treatment, which allowed some of the leaves in all four orthostichies to contribute photosynthate to the clone carbon economy, was less damaging than treatments that rendered some orthostichies completely unproductive. Analyses of the effects of stolon apex removal in G. hederacea have shown that losses in potential biomass production (estimated by comparison with the growth of clones without apices removed) and in several other growth characteristics measured at a primary structural level were almost exactly compensated for by increased growth of secondary branches (Pelling 1994). In contrast, when clones were shaded, or when rooting of ramets was prevented, some growth-related measurements either over- or under-compensated in comparison with untreated clones. Studies of the effects of leachates of G. hederacea on Raphanus sativus a