This section is from the book "An Illustrated Flora Of The Northern United States, Canada And The British Possessions Vol3", by Nathaniel Lord Britton, Addison Brown. Also available from Amazon: An Illustrated Flora of the Northern United States, Canada and the British Possessions. 3 Volume Set..
Perennial simple or sometimes branched herbs, with petioled leaves, and rather small clustered purple or white flowers, in terminal and sometimes also axillary, dense bracted spikes or heads. Calyx oblong, reticulate-veined, about 10-nerved, deeply 2- lipped, closed in fruit; upper lip nearly truncate, or with 3 short teeth; lower lip 2-cleft, its teeth lanceolate. Corolla-tube inflated, slightly narrowed at the mouth, its limb strongly 2-lipped; upper lip entire, arched; lower lip spreading, 3-lobed. Stamens 4, didynamous, ascending under the upper lip of the corolla, the lower pair the longer; filaments of the longer stamens 2-toothed at the summit, one of the teeth bearing the anther, the other sterile; anthers 2-celled, the sacs divergent or divaricate. Ovary deeply 4-parted. Nutlets ovoid, smooth. [Origin of name doubtful; often spelled Brunella, the pre-Linnaean form.]
About 5 species, of wide geographic distribution. Only the following typical one occurs in North America.
Prunella vulgaris L. Sp. Pl. 600. 1753.
Pubescent or nearly glabrous; stem slender, procumbent or ascending or erect, usually simple, but sometimes considerably branched, 2'-2° high. Leaves ovate, oblong or oblong-lanceolate, obtuse or subacute at the apex, usually narrowed at the base, entire, crenate, dentate, or incised, 1'-4' long, the lowest commonly shorter and sometimes subcordate; spikes terminal, sessile or short-peduncled, very dense, 1/2'-1' long in flower, becoming 2'-4' long in fruit; bracts broadly ovate-orbicular, cuspidate, more or less ciliate; corolla violet, purple, or sometimes white, 4"-6" long, about twice as long as the purplish or green calyx; calyx-teeth often ciliate.
In fields, woods and waste places throughout nearly the whole of North America. Native and naturalized from Europe. Native also of Asia. Thimble-flower. All-heal. Brown-wort. Carpenter's-herb or -weed. Hook-heal or -weed. Heart-of-the-earth. Sicklewort. Blue-curls. May-Oct.
Prunella laciniata L., differing by pin-natifid or deeply incised leaves, found near Washington, D. C, and illustrated in our first edition, is a race of this species.