Put oven rack in middle position and preheat oven to 400°F. Butter a 9- by 2-inch round cake pan or cast iron skillet.

Whisk together flours, wheat germ, salt, sugar, baking soda, and cream of tartar in a large bowl until combined well. Blend in butter with a pastry blender or your fingertips until mixture resembles coarse meal. Make a well in center and add buttermilk, stirring until a dough forms. Gently knead on a floured surface, adding just enough more flour to keep dough from sticking, until smooth, about 3 minutes.

Transfer dough to cake pan and flatten to fill pan. With a sharp knife, cut an X (1/2 inch deep) across top of dough (5 inches long). Bake until loaf is lightly browned and sounds hollow when bottom is tapped, 30 to 40 minutes. Cool in pan on a rack 10 minutes, then turn out onto rack and cool, right side up, about 1 hour.

Notes: Bread can be served the day it is made, but it slices more easily if kept, wrapped in plastic wrap, at room temperature 1 day. Leftover bread keeps, wrapped in plastic wrap, at room temperature 4 days. The dough was well-formed after using only 1 3/4 cups of buttermilk. The extra quarter cup would've been too much. I'd advise adding buttermilk a little at a time until the dough is soft, but firm and pliable.