This section is from the book "An Illustrated Flora Of The Northern United States, Canada And The British Possessions Vol1", 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..
Annual or perennial stingless diffuse or erect herbs, with alternate entire 3-nerved petioled leaves, no stipules, and axillary glomerate polygamous flowers, involucrate by leafy bracts. Calyx of the staminate flowers 4-parted or of 4 (rarely 3) distinct sepals. Fertile flowers with a tubular or campanulate 4-lobed calyx investing the ovary, a short or slender style, and a penicillate stigma. Achene ovoid, enclosed by the withering-persistent pistillate calyx. [Ancient Latin, referring to the growth of some species on walls.]
About 7 species, widely distributed; besides the following another occurs in the southern United States and one in western North America. Type species: Parietaria officinalis L.
Annual, pubescent, stem weak, simple or sparingly branched, ascending or reclining, very slender, 4'-15' long. Leaves lanceolate or oblong-lanceolate, membranous, dotted, acuminate at the apex, narrowed at the base, 3-nerved and with 1-3 pairs of weaker veins above, slender-petioled, I' - 3' long, 1/4'-1/2' wide; petioles 1/4"-1" long, almost filiform; flowers glomerate in all except the lowest axils, the clusters shorter than the petioles; bracts of the involucre linear, 2-3 times as long as the flowers; style almost none; achene about 1/2"long.
On dry rocks and banks, Maine to Ontario, British Columbia, Nevada, Tennessee and Mexico. June-Aug. Hammerwort.