1. Preheat oven to 350 degrees Fahrenheit.
2. Combine the pitted plums, lemon juice and sugar in a bowl and let sit for half an hour.
3. Meanwhile, in the bowl of an electric mixer fitted with beaters, beat margarine (or butter) with the sugar until it is the texture of grainy sand. Add the oil and beat on high for three minutes, until pale and smooth. Lower speed and add the flour, baking powder, eggs, orange juice and lemon extract. Beat until pale and smooth, about four minutes.
4. Grease three nine-inch round pans. Add the coriander into the plum mixture and mix again to coat all the plums. Divide the batter amongst the three pans, press in plums cut side down, around the perimeter of the cake, then one row in and place one in the centre. The plums should be close together, but not touching each other. Bake for 55 minutes.