I came across a Horse Chestnut tree (also called Buckeye in some parts) earlier this year at a moment when the spiky green seed pods were at a very intriguing stage in their formation. They looked like what you might imagine a bacteria would look like under a powerful microscope, with an almost alien look. The lighting conditions were such that I was able to take few pictures in which the soft lighter green spikes really stood out against the background. Perhaps these images speak for themselves without further explanation.
Some of the little spiky balls had just formed from the flower stage and were not much larger than a marble.
I also got a couple of interesting shots of the Horse chestnut leaf margin (below) and the leaf stem.
In this image one can see how the Horse chestnut palmate leaf attaches to the stem. This is one good way to help distinguish this tree from the true Chestnut Castanea sativa.