The third image is a smaller version of the first with a red line at the upper left. This line represents the location of a longitudinal section seen in the last image (not the exact same one). In the bottom image the outer surface is on the right, and as you move left you move into the stem. If you compare the top image to the bottom you can visualize the part you are looking at, as cut from the side. You can clearly see a cross-cut through the xylum/phloem. In the xylum you can see the lignin, the structural material that provides support and strength. The lignan is the spiral-like "bands" on the red "tube" (xylum) to the left of center. If you look carefully at the xylum tubes in the second image you can a small piece of the lignan that was not completely cut through... it is tough stuff!
Very nice.