6. Penthoraceae Rydberg ex Britton
Craig C. Freeman
Perennial herbs. Stems simple or branched. Leaves cauline, alternate, simple; stipules absent; petiole present or absent; blade margins sharply serrate or doubly serrate. Inflorescences terminal, scorpioid racemes. Flowers bisexual; perianth and androecium hypogynous or perigynous; hypanthium present; sepals 5(-8), connate proximally; petals absent or 1-8, distinct; nectary disc absent; stamens 10, distinct, adnate to rim of hypanthium; anthers dehiscent by longitudinal slits; pistils 1, (4-)5(-8)-carpellate, connate basally and laterally, partially adnate to hypanthium; ovary superior or partially inferior, (4-)5(-8)-locular proximally; placentation marginal; ovules anatropous, bitegmic, crassinucellate; styles 1 per carpel, distinct; stigmas 1 per carpel, terminal, capitate. Fruits capsular, dehiscence circumscissile proximal to free portion of styles. Seeds 300-400, tan, yellowish brown, or pinkish, ellipsoid to subfusiform, funicular end more tapered; embryo straight; endosperm scant, cellular.
Genus 1, species 2 (1 in the flora): North America, Europe (Russia), Asia.
The position of Penthorum within Rosales has been disputed extensively. A. Cronquist (1981) considered it to be transitional between Crassulaceae and Saxifragaceae. He included it in Saxifragaceae, stating that Penthorum was not distinct enough from Crassulaceae and Saxifragaceae to warrant being treated as a distinct family. Placement of the genus by others has depended on the morphological, anatomical, and embryological traits emphasized. Molecular studies suggest that the genus is sister to Haloragaceae (D. R. Morgan and D. E. Soltis 1993; D. E. Soltis and P. S. Soltis 1997). Recent authors often have placed it in the monogeneric Penthoraceae.
SELECTED REFERENCES Baldwin, J. T. and B. M. Speese. 1951. Penthorum: Its chromosomes. Rhodora 53: 89-91. Haskins, M. L. and W. J. Hayden. 1987. Anatomy and affinities of Penthorum. Amer. J. Bot. 74: 164-177. Spongberg, S. A. 1972. The genera of Saxifragaceae in the southeastern United States. J. Arnold Arbor. 53: 409-498.
