I want to scatter leaves on terrain using a particle system. The leaf-object is a plane whose surface is parallel with the terrain. My assumption was that the scattered leaves would therefore "lay flat" on the terrain. But they don't, rather they are turned 90 degrees on edge, i.e. standing vertical. They are half-above and half-below the terrain, which is consistent with origin at center of plane. I've tried rotating the source object X, Y and Z each by 90%, but that doesn't solve it.
Note that origin of source object is centered on the plane, but slightly below in z-axis to allow for small rotation from the "normal" emission axis.