1) Preheat the oven to 350°F (180°C).
2) In a saucepan, simmer the Saskatoons in water and lemon juice for 10 minutes.
3) Stir in the sugar until dissolved and allow to cool.
4) Stir in the lemon rind and tapioca.
5) In a 9 inch pastrylined pie plate, pour the mixture and dot with butter.
6) Cover with top pastry crust, seal the edges and bake in the preheated oven for 45-50 minutes.