Myriophyllum verticillatum L.
Family: Haloragaceae
whorled water-milfoil
Myriophyllum verticillatum image
Steve C. Garske  
Etymology: Myriophyllum: from Greek myrios, "numberless," and phyllon, "leaf," alluding to the many divisions of the submerged leaves of these aquatic plants
Plants: perennial, submersed aquatic; stems greenish-brown
Leaves: whorled, stalkless; finely pinnately-divided into 7-10 thread-like, paired leaflets
Flowers: 4-parted, club-shaped winter buds; flowers from the axils of the sharp, deeply-lobed, whorled bracts