Crate cruncher, I'm afraid it is you who fails to understand. Check out http://members.eaa.org/homebuilders/...orkBasic.,html
or Wood Design Worksheet-EAA-The Spirit of Aviation-Oshkosh,WI which shows by actual tests that the shear strength for wooden beams in nearly double for Spruce and Mahogany when the grain is at 45 Degrees to the span versus vertical or horizontal. If you still doubt that, just Google "Aircraft Shear Webbing grain orientation" and check out the some 1000 plus topics that will turn up substantiating this. The original question for this item in the forums was "What is the effect of horizontal versus vertical grain on shear webs". All valid discussions by the experts show that for maximum shear strength, the grain should be at a 45 degree angle to the span.