The old leaf can be discarded. Plants which root most readily from leaf cuttings include African Violets and Sansevieria. Leaf cuttings of African violets root so readily, they can simply be suspended in a well aerated, jar of water. The suspended leaves can be supported by simply covering the mouth of a jar with foil or paper held in place with a rubber band. Holes are easily punched in this covering, and the leaf stems inserted so the bottom of each leaf stalk touches the water.
Sansevieria is another interesting plant that can be started from leaf cuttings.
The leaves are long, leathery and sword-shaped. Just select a whole leaf and then cut it into 2-inch sections starting from the tip all the way down. Leaf cuttings can be literally crowded together, almost shoulder to shoulder. This crowding will not harm them, and once the root systems have been developed they can be separated for transplanting into individual pots. The bottom portion of the vein can then be pressed into the propagation medium with the leaf portion sticking up to root just like a leaf cutting.