Preheat oven to 450 degrees F. In a large bowl stir together flour, baking powder, sugar, salt, and cream of tartar. Using a pastry blender, cut in butter until mixture resembles coarse crumbs. Make a well in center of flour mixture. Add buttermilk all at once. Using a fork, stir just until moistened.
Turn dough out onto lightly floured surface. Knead by folding and gently pressing dough 4 to 6 strokes, just until dough holds together. Pat or lightly roll to 1/2 inch thick. Cut dough with floured 1-1/2-inch round, oval, or flower cookie cutters, rerolling scraps as needed. Place biscuits 1 inch apart on ungreased baking sheet; bake for 8 minutes.
Meanwhile, in a small bowl combine egg white and water. Brush biscuits with egg white mixture. Arrange Signet marigold flowers and leaves on biscuit tops. Brush again with egg-white mixture. Return to oven; bake for 3 minutes more or until golden.
Remove biscuits from sheet; cool on wire rack. Makes about 38 biscuits.