The nodules on the plane trees give them an unusual knobbly shape. They are also unusual when viewed at high magnification in the scanning electron microscope. This SEM photo shows a rather messy, chaotic structure…
…which is very different from that of normal wood. For example here’s a picture Clodagh took of a twig from the Oregon maple, at the same magnification:
This remarkably regular pattern of cells makes the wood very strong and light. The nodule is also made of cells, but they have grown in a chaotic, random pattern, creating material which is much weaker. This explains why these trees have made very thick trunks to support their weight. Nodular wood is much sought-after by woodturners because it makes a beautiful patterned surface when made into, for example, a bowl, but they know to treat it very carefully because it breaks easily.
Not all the wood in the plane trees is chaotic though. Clodagh found this area where the tree has managed to get back to something approaching a regular cellular structure: