This section is from the book "Nature's Garden", by Neltje Blanchan. Also available from Amazon: Nature's Garden; An Aid To Knowledge Of Our Wild Flowers And Their Insect Visitors.
Flowers - Light bluish purple, dotted with small specks of reddish violet; growing singly or in clusters along stem, seated in leaf axils; calyx hairy, with 5 sharp teeth; corolla tubular, over 1/2 in. long, 2-lipped, the upper lip 2-lobed, lower lip with 3 spreading lobes, middle one largest; 4 stamens in pairs under upper lip; the anther sacs spreading; 1 pistil with 2-lobed style. Stem: Trailing, rooting at intervals, sometimes 18 in. long, leafy, the branches ascending. Leaves: From 1/2 to 1 1/2 in. across; smooth, rounded, kidney-shaped, scallop-edged.
Preferred Habitat - Waste places, shady ground.
Flowering Season - March - May.
Distribution - Eastern half of Canada and the United States, from Georgia and Kansas northward.
Besides the larger flowers, containing both stamens and pistils, borne on this little immigrant, smaller female flowers, containing a pistil only, occur just as they do in thyme, mint, marjoram, and doubtless other members of the great family to which all belong. Muller attempted to prove that these small flowers, being the least showy, are the last to be visited by insects, which, having previously dusted themselves with pollen from the stamens of the larger flowers when they first open, are in a condition to make cross-fertilization certain. So much for the small flower's method of making insects serve its end; the larger flowers have another way. At first they are male; that is, the pistil is as yet undeveloped and the four stamens are mature, ready to shed pollen on any insect alighting on the lip. Later, when the stamens are past maturity, the pistil elongates itself and is ready for the reception of pollen brought from younger flowers. Many blossoms are male on the first day of opening, and female later, to protect themselves against self-fertilization.
In Europe, where the aromatic leaves of this little creeper were long ago used for fermenting and clarifying beer, it is known by such names as ale-hoof and gill ale - gill, it is said, being derived from the old French word, guiller, to ferment or make merry. Having trailed across Europe, the persistent hardy plant is now creeping its way over our continent, much to the disgust of cattle, which show unmistakable dislike for a single leaf caught up in a mouthful of herbage.
Very closely allied to the ground ivy is the Catmint or Catnip (Nepeta Cataria), whose pale-purple, or nearly white flowers, dark-spotted, may be most easily named by crushing the coarsely toothed leaves in one's hand. It is curious how cats will seek out this hoary-hairy plant in the waste places where it grows and become half-crazed with delight over its aromatic odor.