For the topic we started on... Chapter 4 of the wood handbook starting at 4-28 is a good read, really the whole section on Natural Characteristics Affecting Mechanical Properties is worth reading. Notice fig 4-4.
The slides on the NCSU site covering softwood rays and resin canals are good too.
While I was looking at those slides another couple of wood tech things came to mind. First as you look at rays and tracheids you are looking at the plumbing of a tree, tracheids, vertical softwood cells, take care of up and down movement. Rays take care of the heart to bark transport among other duties.
Second, you probably know that wood shrinks about half as much radially as it does tangentially. While looking at the rays, notice the grain direction they have within the wood. They don't want to shrink lengthwise and so reduce shrinkage in the radial direction. The microfibrils winding around the pit pairs on the radial faces of the cell wall are another reason.
