Watsonia, 14, 263-272 (1983). 263 Variation in the Juncus bufonius L. aggregate in western Europe T. A. COPE - Royal Botanic Gardens, Kew, Richmond, Surrey and T C. A. STACE Department of Botany, University of Leicester ABSTRACT The characters which have been used in the delimitation of segregates of the Juncus bufonius L. aggregate are reviewed, and the pattern of variation exhibited by them within and between segregates is described. The great plasticity in some characteristics is demonstrated by growth experiments. The characters most valuable from a taxonomic point of view are those which are relatively non-plastic, show distinct discontinuities, and are correlated with others. These include seed shape and testa sculpturing, anther/filament ratio, shapes and relative lengths of capsule and inner tepals, and (to a lesser extent) inilorescence form. These characters, together with chromosome number, geographical range and a few other morphological features, indicate that J. ambiguus Guss. , J. foliosus Desf. , J. hybridus Brot. and J. sorrentinii Parl. should be recognised as species distinct from J. bufonius sensu stricto, although the last even then remains a complex and variable taxon which often approaches ` closely examples of the other four segregates. INTRODUCTION The genus Juncus L. consists of about 300 species ranging from dwarf, ephemeral annuals to large tufted or creeping rhizomatous perennials. A number of infrageneric treatments have been proposed (e.g. `Buchenau 1890, 1906; Krechetovich & Goncharov 1935; Snogerup 197la, 1971b, 1972) and the tendency today is to divide the genus into nine subgenera or sections. Annual species with grass-like leaves and a diffuse, leafy, terminal inllorescence, including J. bufonius L. , comprise subgenus Poiophylli Buchenau. Sometimes this subgenus is taken to include the perennial species now generally separated as subgenus Pseudotenageia Krech. & Gonch. When described by Linnaeus (1753), J. bufonius consisted of five varieties. Since then about 60 names that can be ascribed to the J. bufonius aggregate have been published, to which can be added a few nomina nuda and a large number of new combinations at various taxonomic levels based on these names. The complexity of the taxonomy of the aggregate is best illustrated by the almost universal disagreement amongst monographers and Flora writers (surveyed in more detail by Cope & Stace (1978)) as to how many taxa there are, at what rank they should be recognised, and which name should be applied to each. Cope & Stace (1978) recognised five segregates within J. bufonius agg.: J. bufonius sensu stricto, J. foliosus Desf., J. ambiguus Guss. , J. hybridus Brot. and J. _ sorrentinii Parl. J. bufonius agg. is morphologically extremely variable. It consists of small green tufted annuals up to 40 cm high with llattish, convolute or subterete leaves up to 15 cm >< 0-5-5 mm. The inliorescence is a compound, bracteate, dichasial cyme (often termed an anthela), and may bear widely spaced to loosely or densely clustered flowers. The outer tepals are acute to long-acuminate or cuspidate and are 4-9(-11) mm long. The inner tepals vary from acute or subacute to rounded or truncate and, in the latter extreme, may also be emarginate and mucronate; they are shorter than the outer tepals and longer than, equal to or shorter than the capsule. The capsule is 3-5~5 >< 1-2-2 mm, oblong, acute to truncate at base and apex, and trilocular. The seeds are 0-3-0-5 >