Make lemon cake mix according to package directions. Use greased 9 x 13 pan. Over unbaked cake mixture put 3 cups raw rhubarb, cut up. Sprinkle 1/2 cup white sugar over this. Pour 1 1/2 to 2 cups whipping cream over sugar. Bake for 40-45 minutes at 350 degrees. Serve with Cool Whip.