Place the green beans, wax beans, carrots and turnips into a saucepan and add enough water to cover the vegetables. Lightly salt the water, and bring to a boil. Cook for about 1/2 hour, then add the potatoes and cook for another 1/2 hour. Stir in butter and if desired, cream.
Mix together the flour and 1/2 cup water, and pour into the soup. Cook for a few more minutes to thicken. Remove from heat and serve hot.