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..
Sium cicutaefolium Schrank, Bair. Fl. 1: 558.
Erect, stout, branched, 2°-6° high. Lower leaves long-petioled, the uppermost nearly sessile; petioles sheathing at the base; leaf-segments 7-17, linear, or lanceolate, 1 1/2'-5' long, 1 1/2-12" wide, acuminate at the apex, sharply serrate, or the lowermost pectinately dissected; umbels 2?-$' broad, 8-20-rayed; rays 1/2'-1 1/2' long; fruit ovate, compressed, about \\" long, the ribs promi-Inent.
In swamps, Nova Scotia to British Columbia, south to Florida, Louisiana and California. Very variable in leaf-form. July-Oct.
Sium Carsoni Durand; A. Gray, Man. Ed. 5, 196. 1867.
Stem slender, weak, 1°-2° long. Leaf-segments 3-7, those of the upper leaves linear, or lanceolate, acute, or acuminate, 1'-2' long, 1V-3" wide, sharply serrate; lower leaves often floating and very thin, the segments broader and laciniate, or dissected; umbels 1'-2' broad, 7-15-rayed; rays 6"-12" long in fruit; fruit somewhat smaller than that of the preceding species.
In streams, Maine, Massachusetts and Rhode Island to Pennsylvania. Perhaps an aquatic race of Sium cicutaefolium, but appearing distinct in Pennsylvania. July-Aug.