HERACLEUM MANTEGAZZIANUM SOMMIER& LEVIER

Biennial or perennial monocarpic herb. Root pale yellow, bruising brown and exuding a sickly yellow sap from the cut surface; tap-root, 45-60 cm, initially narrow, becoming thicker and more divided with age, the crown reaching up to 15 cm in diameter at flowering. Lateral branches spreading horizontally or spirally from the often multiple tap-root; laterals suppressed in dense stands. The exocortex of the main root contractile from the first seedling leaf stage, the crown of adult plants ultimately positioned 8-12cm below ground level, deeper where sediments have accumulated after flooding. Older plants develop a solid stem-like stock, with old leaf scars, between the true root and the stem base. Stem single, annual, up to 0cm thick at base and 200-500cm tall, hollow, ridged, with purple blotches decreasing towards the top. Stem and petioles clothed with pustulate bristles, dense in the young state, more sparse after elongation. Leaves alternate, lower up to 250-300 cm, blades ternately or pinnately divided to a varying extent, coarsely and irregularly deeply lobed: lobes and larger teeth long-acuminate, usually puberulent below and more or less glabrous above; petiole stout, fleshy, hollow, more or less hairy, shortly sheathing at the base. Upper leaves progressively smaller, with a greatly inflated sheathing petiole to almost sessile, becoming increasingly bracteate higher up the stem. Flowers in compound umbels up to 80 cm across with 50-150 somewhat unequal hairy rays (15-40 cm). Rays shortest in the centre and longest at the edge of the umbel bringing all secondary umbels to a similar subspherical plane. Central ray vertical and distinct in main and branch terminal umbels, the remainder progressively curved centrifugally. Compound umbels on a short stem-like hollow peduncle, subtended by bracts. The terminal umbel is largest and hermaphrodite, surrounded by up to eight satellite umbels on elongated curving stalks raising them up to 40cm above the level of the terminal umbel. Additional terminal umbels may be borne on the main axillary branches which develop from the base of the main stem upwards; however, other umbels can develop from any of the remaining leaf or bract axils, especially on large vigorous plants or where the growth point of the main stem becomes damaged. Under stress, umbels may be degenerate and sterile. Umbels mature in sequence, the lower umbels smaller and sometimes male. Bracts usually several, linear or ovate and long acuminate; bracteoles linear, caducous. Pedicels 10-20 mm, hairy. Flowers white or rarely pinkish, sepals triangular, calyx teeth prominent, acute. Petals up to 12mm, outer petals radiating, styles with enlarged base forming the stylopodium. Styles about three times the length of the stylopodium, divergent or somewhat recurved. Stigma capitate. Pollen grains range from 56 to 77 ,m long (Grace & Nelson 1981). Fruit elliptical 6-18mm long, 4-10mm wide, typically 15mm long, 8 mm wide in large fruits, 8 mm long, 6 mm wide in small fruits, e.g. from the satellite inflorescences. Fruits usually glabrous to villous, dorsally much compressed; commissure broad. The fruit splits into two winged mericarps. Mericarp with slender, low dorsal ridges and rather broadly winged lateral ridges; the wings are flat and closely appressed to one another; carpophore present. The mericarp has three to five, usually four, conspicuous solitary claviform vittae (oil ducts), dark brown on the outer wall, approximately two-thirds as long as the mericarp and usually 0.51.0mm wide at the lower end (Tutin 1980) and two smaller vittae on the inner wall. The endosperm is oily and mature fruits have a strong resinous smell. Mericarp mass variable, ranging from a mean dry weight of 5.7mg (Grime et al. 1981) to 11.9-12.7mg dry weight from a population in Sheffield (J. Hodgson personal communication), 16.5mg the average dry weight in a population in the Lea Valley, London (range 4.6-23.2mg) to 18.2mg the average weight from a terminal umbel of a plant in west Scotland. The seed size and weight depend on the vigour of the plant and hierarchy of the umbel and are normally largest in the terminal umbel. Size and weight, but not necessarily viability, are reduced in the satellite and lower order umbels and in those late developing. The cotyledons are abruptly contracted into a petiole. *Abbreviated references are used for many standard works: see Journal of Ecology (1975), 63, 335-344. Nomenclature of vascular plants follows Flora Europaea and, where different, Stace (1991). ? 1996 British Ecological Society