I love the skeleton that is revealed as the leaves fall, the way the network of branch upon branch seems to grow upwards and outwards like fingers reaching for the sky.
I am also fascinated by the way nature mirrors itself. Seeing the hidden structure under the foliage seems to me to replicate the networks of vessels and nerves in the human body. The same sense of an unseen system, forming the roots and branches of life. This picture on display at the Museum of Science and Industry recently, albeit not particularly autumnal, shows the wiring of the human brain and goes some way to depicting what I mean.