The Collaborative International Dictionary of English v.0.48:

Hemlock \Hem"lock\, n. [OE. hemeluc, humloc, AS. hemlic,
   hymlic.]
   1. (Bot.) The name of several poisonous umbelliferous herbs
      having finely cut leaves and small white flowers, as the
      Cicuta maculata, Cicuta bulbifera, and Cicuta
      virosa, and the Conium maculatum. See Conium.
      [1913 Webster]

   Note: The potion of hemlock administered to Socrates is by
         some thought to have been a decoction of Cicuta
         virosa, or water hemlock, by others, of Conium
         maculatum.
         [1913 Webster]

   2. (Bot.) An evergreen tree common in North America (Abies
      Canadensis or Tsuga Canadensis); hemlock spruce.
      [1913 Webster]

            The murmuring pines and the hemlocks. --Longfellow.
      [1913 Webster]

   3. The wood or timber of the hemlock tree.
      [1913 Webster]

   Ground hemlock, or Dwarf hemlock. See under Ground.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Water hemlock \Wa"ter hem"lock\ (Bot)
   (a) A poisonous umbelliferous plant (Cicuta virosa) of
       Europe; also, any one of several plants of that genus.
   (b) A poisonous plant ([OE]nanthe crocata) resembling the
       above.
       [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Cowbane \Cow"bane`\ (kou"b[=a]n`), n. (Bot.)
   A poisonous umbelliferous plant; in England, the Cicuta
   virosa; in the United States, the Cicuta maculata and the
   Archemora rigida. See Water hemlock.
   [1913 Webster]