This section is from the book "An Illustrated Flora Of The Northern United States, Canada And The British Possessions Vol2", 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..
Glabrous slender branching annuals, the lower and basal leaves mostly linear and entire, those of the stem mainly divided into few linear segments. Involucres and involucels of several subulate or narrowly linear bracts, sometimes deciduous. Flowers small, white, in terminal and lateral compound umbels. Calyx-teeth short, persistent. Fruit ovoid, or oblong, nearly terete, glabrous, strongly ribbed, the lateral ribs the larger; oil-tubes solitary in the intervals and 2 on the commissural side of each carpel. Seed-face flat. Stylopodium conic. [Greek, dog-celery.]
Two known species, natives of the southern United States, the following typical.
C. pinnatum DC. Mem. Omb. 45 pl. 11. f. B. 1829.
Stem erect, or assurgent, 1°-2° high. Lower and basal leaves petioled, the blades elongated-linear, entire, acuminate or acute at each end, 1'-3' long, 1 1/2'-3' wide; stem-leaves pinnately divided nearly to the midvein into 3-9 narrowly linear entire segments, the terminal segment much larger than the lateral ones, or some of them entire; bracts of the involucres 2"-3" long; umbels 4-10-rayed; rays very slender, 1/2'-1 1/2' long; fruit about 2" long, less than 1" wide, tipped by the conic stylopodium and crowned by the ovate calyx-teeth.