Platanthera Rich.

Small to medium-sized herbs. Rootstock tuberous, or composed of a fascicle of fleshy, tapered roots, tubers (when present) entire, fusiform or ovoid. Stem erect, leafy or not. Leaves two or three at the base and/or cauline, unspotted. Inflorescence cylindrical, usually rather sparsely flowered (rarely many-flowered), sometimes dense; floral bracts leafy. Flowers medium sized or small, greenish, white, orange, yellow, or rose-pink, often sweetly scented. Dorsal sepal and petals usually connivent, forming a hood. Lateral sepals free or lower margins partly adnate to base of lip, spreading or recurved. Labellum entire and strap-shaped, divided or fimbriate to varying degrees, sometimes with a central thickening, spurred, spur varying in length, filiform or narrowly cylindrical, sometimes clavate above the middle, the orifice situated below the stigma, sometimes with a wide and deep nectary entrance created by expanded margins of the labellum base, which adhere to the sides of the column. Colrmm short, rather truncate; rostellum flattened, triangular, with a rudimentary m edian lobe; anther broad, immobile, loculi more or less separated by connective tissue or tl1e stigma, either divergent or convergent and parallel to varying degrees; pollinia two, clavate or each cleft into two halves, with caudicles attached to naked viscidia, bursicles absent. Ovary cylindrical to fusiform, curved toward apex, sessile, twisted, glabrous. (JW).

The terrestrial genus Platanthera usually grows on damp and acidic soils, although sometimes it occurs on drier and/or calcareous substrates. Platanthera hyperborea is reported to grow in permanently wet and acidic soils of swamps and marshes across its range in the USA (Hitchcock et al. 1969), Europe, and Asia (Hulten 1927; Delforge 1995). However, it occasionally grows on dry grass land, as in the high mountain regions of Japan (Ohwi 1965; Delforge 1995) and according to Hitchcock et al. (1969) occurs in the Pacific Northwest of the US in bogs where the soil can be of calcareous origin. Other Platanthera species tolerant of a range of different soil types include P. bifolia, which usually grows on damp acidic or neutral substrates, as in the heaths and moorlands of Scotland, but occurs less frequently on calcareous soils such as the chalk downs of southern England (Summerhayes 1951; Allan el al. 1993). Likewise, in Finland P. bifolia plants have been reported to grow in both the dry and open habitats of the archipelagos and coastal areas and in the wetter and shadier habitats of woodland and forests (Haeggstrom 1994). The closely related P. chlorantha differs markedly in its ecological preferences. It typically grows on calcareous and alkaline soils and in warmer and drier conditions than P. bifolia and is consequently more abundant in southern England. l lowever, it also grows in a variety of other habitat such as more acidic heathlands (Summerhayes 1951), and sometimes the two species can be found growing together in the same habitat (Summerhayes 1951; Nilsson 1983/J).
Platanthera grows in a variety of habitat types including marshes on taiga and tundra, moors and heaths, sphagnum and peat bogs, swamps, shrubland, open deciduous woodlands and coniferous forests. Consequently Platanthera species are subject to varying light conditions, from full sun to dense shade. Some commonly occur in open habitats but can tolerate lower light levels. For example, P. bifolia in Britain is a common plant of heaths and moorlands, growing among such species as heather (Calluna vulgaris Salisb.), cross-leaved heath (Erica tetralix L.), and mat grass (Nardus stricta L.). It also occurs in bogs with sedges (Carex species), sweet gale (Myrica gale L.), and the common marsh orchid (Dactylorhiza praetermissa) and in upland meadows and fields with other orchids including D. fuchsii, D. maculata ssp. ericetorum, Gymnadenia conopsea, and P. chlorantha. In all of these habitats, P. bifolia receives comparatively high light levels, but it can also grow in shaded environments, such as beechwoods in southern England where it occurs with Listera ovata and other woodland orchids (Summerhayes 1951; Allan et al. 1993). Platanthera chlorantha is a more typical woodland species (Davies et al. 1988). Summerhayes (1951) noted that it is characteristic of coppiced oak woods of southern England where it occurs with Orchis mascula, L. ovata, and D. fuchsii, whereas in Suffolk it occurs in a number of ancient woods growing on chalky boulder clay (Sanford 1991). Elsewhere in England, it is found in scrubland where it is shaded by bushes and small trees and often co-occurs with Cephalanthera damasonium and O. insectifera when on chalk (Summerhayes 1951). In Scotland its most usual habitat is open pasture and grassland (Allan et al. 1993). By comparison, in the United States many Platanthera species grow in deep sphagnum and mud in forest areas comprising conifers such as tamaracks, cedars, balsams, and firs (Luer 1975). Platanthera orbiculata (Pursh) Lindl. is widespread in the forests of North America including coniferous swamps, and P. macrophylla (Goldie) P. M. Brown is found in mesic areas of the eastern mixed forest (Reddoch and Reddoch 1993). Platanthera hyperborea is said to tolerate deep shade in its woodland habitat (Hitchcock et al. 1969), although P. grandiflora (Bigelow) Lindl. occurs in full sun to shade in the Appalachian Mountains, frequently under or near conifers (Stoutamire 1974). Flowering of some Platanthera species growing in woodlands can be suppressed by dense shade. Both P. bifolia and P. chlorantha, for example, tend to be distributed in gaps in woodlands, and their frequency of flowering is increased following coppicing or felling of trees (Summerhayes 1951; Sanford 1991). Platanthera species grow at a range of elevations from lowlands to mountain regions. The North American species P. grandiflora exemplifies the range that can be achieved by some individual species. In the north of the United States, P. grandiflora grows at sea level in the New England states, but farther south it is restricted to the high elevations of the Appalachian uplands (Stoutamire 1974). Similarly, the widespread P. hyperborea occurs both in low-lying marshes and bogs (Hulten 1927) and dry alpine grassland up to 3000 m, although it is far less common at high elevations (Delforge 1995; Ohwi 1965). In Britain, P. chlorantha more lowland in distribution than P. bifolia (Summerhayes 1951 ).
Flowering of Platanthera normally takes place between late May and early August (Davies et al. 1988), the exact period varying among species and influenced by factors such as microclimate and the elevation and latitude of individual populations. ln Britain P. chlorantha usually flowers in June to July, but in an early season or in sheltered localities it may start to flower in May (Summerhayes 1951). Also, P. chlorantha usually commences flowering 2-3 weeks before P. bifolia (Delforge 1995). During the flowering period, the leaves of Platanthera species remain green ('summer-green' orchids), but they senesce in the autumn, and the plants overwinter as underground tubers.
The number of flowering plants in Platanthera populations fluctuates from year to year, a feature that has been attributed to seasonal climatic variations. During their 10-year study of a P. chlorantha population in southern Scotland Dickson and Parkes (1994) observed that fewer plants flowered following years when rainfall levels between April and August were particularly low and that there was premature floral senescence of flowering plants during dry summer months. According to Summerhayes (195 1), a large proportion of plants in many P. chlorantha populations in England do not flower in most years, although they may persist in a vegetative form for a lo n g period until conditions a re more favourable for flowering stems to be produced. Aerial stems in Platanthera develop in springtime from overwintered buds formed in the preceding growing season and utilize food stored in the tuber also produced that year. Usually a new bud and tuber develop each year as replacements. They form at the base of the aerial stem during the latter half of the growing season, the tuber is already well developed by the time the flowers open (Summerhayes 1951) . Sheviak (1990) noted for Platanthera species, including P. hyperborea, that if the bud or shoot is accidentally destroyed or damaged by late frost , foraging animals, disease, or inappropriate management practices, the orchid is unable to recover and usually dies. He contrasted this feature of Platanthera to that of other orchids including Cypripedium species, in which similar damage may simply result in suppression of flowering capacity for two or more years. Since normally only one new bud and tuber are formed per year per plant, and since Platanthera species have no other means of vegetative propagation, increase in population size by vegetative growth rarely occurs (Summerhayes 1951). One exception is P. obtusa, an arctic/subarctic species, which commonly develops more than one tuber each season. This must be of adaptive advantage in an environment in which the short flowering season could make seed production very irregular (Hesselman 1900, cited in Rasmussen 1995). Most other Platanthera species, however, are likely to be entirely dependent on seed dispersal for increasing population size and colonization of new habitats.
Fruit production in Platanthera species is often high with average fruit-set figures of between 50% and 60% recorded for P. ciliaris Lindl. and P. stricta Lindl. in the USA and P. chlorantha in Sweden and 90% for P. mandarinorum Rchb.f. subsp. hachijoensis (Honda) Hiroe in Japan (Neiland and Wilcock 1998). Reports of lower fruit-set levels are unusual, for example less than 30% in two Platanthera species from Japan (lnoue 1985, 1986). For the rare North American species P. integrilabia (Correll) Luer, the low fruit-set of 13.6% in one population may be explained by low levels of self-pollination in the absence of pollinating insects (Zettler and Fairey 1990). A higher fruit-set of 56.9% was achieved in a large population of this rare orchid in Tennessee where pollinators were seen to be present (Zetder et al. 1996). Plants in this population produced a mean of 4.7 capsules per inflorescence. With each capsule containing 3433 seeds, the average seed output per plant was estimated to be c. 16 000 (Zettler et al. 1996). Salisbury (1942) counted 2500 seeds in one capsule of P. chlorantha and calculated that the average output of seeds per plant could be 13 750 for a population in England. A count of the ovule number per ovary in a P. chlorantha flower revealed 6280 ovules (Neil and 1994), indicating that average seed production per flower is variable and that output per plant may be higher than expected b y Salisbury. Spread of the species by seed dispersal seems to be particularly effective in Platanthera. In the United States Platanthera species are opportunistic colonizers of roadside verges and excavations (Sheviak 1990; A. L. Jesup, cited in lUCN Orchid Specialist Group 1996) and may form large populations in suitable habitats (e.g. >2000 plants in the study by Carroll et al. 1984).
Most germination of Platanthera seeds in the field probably takes place in the spring after seed dispersal (Rasmussen 1995), but it may extend throughout the year due to seed dormancy mechanisms, with germination being triggered by some environmental signal such as a period of low temperature (Gregg 1990 , cited in Rasmussen 1995). Observations made by Fuchs and Ziegenspeck (1927) indicated that the first apical bud is formed from the protocorm in the autumn and that it produces the leafy shoot the following spring. Shoot formation is followed by emergence of a root from the upper part of the mycorrhizome, which then becomes infected with mycorrhiza. The plant initially overwinters as a short rhizome but in later seasons, at least three years after germination, the first tuber is produced. Some mycorrhizal fungi have been isolated from the roots of Platanthera species and identified as Leptodontidium orchidicola from P. orbiculata and P. hyperborea (Currah et al. 1987, 1990), Rhizoctonia anaticula and Sistrema species from P. obtusa (Banks & Pursh) Lindl. (Smreciu and Currah 1989), Sebacina vermifera from P. orbiculata (Currah et al. 1990), Rhizoctonia species from P. integrilabia (Correll) Luer (Zettler and Fairey 1990), Tulasnella calospora from P. bifolia (AIIan et al. 1993), Ceratorhiza pernacatena from P. praeclara C.J.Sheviak & M.L.Bowles (Zelmer and Currah 1995), and Epulorhiza inquilina from P. clavellata (Michx.) Luer, P. cristata Lindl., and P. integrilabia (Zettler and Hofer 1998). Some of these fungal symbionts have successfully promoted seed germination and protocorm development of Platanthera species in experimental trials. lt has been suggested that in vitro symbiotic seed germination techniques could be applied to the conervation of rare Platanthera orchids by producing seedlings, which could be transplanted to field locations (Zettler and Mclnnis 1992), especially because germination of Platanthera seeds through asymbiotic methods can be slow (Rasmussen 1995).
Several members of the genus are rare orchids, and some are protected by national laws, e.g. P. integrilabia in the United States (Zettler et al. 1996) and P. obtusata ssp. oligantha (= P. oligantha Turcz.) in Norway and Sweden (Hoiland 1990), and may be the focus of active conservation measures. Other more common and widespread Platanthera species may be threatened locally by collecting (P. chlorantha is collected for use in herbal medicine in China; Chen and Tang 1982), by habitat change including that due to ecological succession, or by different management regimes such as cessation of coppicing or replanting of woodlands, which can detrimentally alter the vegetation composition and light levels of Platanthera habitats (IUCN Orchid Specialist Group 1996). (RN).

Approximately 200 species (Luer 1975) distributed over Europe and North Africa, extending eastward across Asia to New Guinea, North and Central America. (GW).