Herbaceous plants whose main stems remain green and don't turn woody with age are the best plants to propagate in water. Not all such plants will grow roots in water. Experiment with plants from your own garden and you may find that many can be propagated with only plain water and a little sunlight. Add aquarium charcoal to the glass of water to keep bad-smelling bacteria from growing. This bacteria can be fatal to the cuttings.
All varieties of basil will grow roots when a cutting is placed in a glass of water. Remove the lower leaves so that no leaves are under water. Make sure the leaf node (the bump on the stem where the leaves used to be) is underwater. Set the container in a sunny window. Roots will form at the leaf nodes in about 10 to 14 days.
Tomato plants send up suckers. These are vigorous upright branches that grow from the crotch where branches grow out from the main stem. These suckers should always be pruned off of the mother plant. Remove the lower leaves and put the sucker into a glass of water, making sure that the remaining leaves stay above the surface of the water. Put the glass in a sunny window and roots will form at the denuded leaf node in about two weeks.
Ivy and Other Houseplants
Ivy is perhaps the most well known houseplant that will root in water, but many other will, such as philodendron, arrowhead plant, and some smaller varieties of dieffenbachia. Remove lower leaves from the stems and insert the cuttings in a glass of water, making sure that the leaves are not under the surface of the water. Roots will form at the leaf nodes in about two to three weeks.