Remove the stalks and put the washed plums into a pan with the water. Sweet, juicy plums will need only ¼ pint water, but the hard cooking variety will need about ¾ pint. Simmer gently until the fruit is well broken down. Stir in the sugar and cook over a low heat, then boil rapidly. Remove the plum stones as they rise to the surface. When setting point is reached, remove from the heat, skim and pour into hot sterilized jars. Cover tightly. If liked a few blanched plum kernels can be added with the sugar.