Quaking Aspens (Populus tremuloides) are one of my favorite trees with their lovely bright green or yellow leaves and striking white bark, but perhaps their most appealing features are their enigmatic "eyes".
Aspen eyes are actually dark markings on the main trunk where side branches used to be. Quaking Aspens have tall, straight, mostly unbranched lower trunks which they achieve through rigorous self-pruning.
Quaking Aspens are shade-intolerant, and as these trees grow taller, they are in a constant race upwards for the sun with their close neighbors in the aspen grove. Their lower branches eventually become shaded, so the Quaking Aspens cut off the flow of sap to these lower branches, causing them to atrophy and fall off, leaving a dark eye where each self-pruned branch used to be. This allows the trees to only expend their limited energy on the more productive, sunlit upper branches.
Aspen eyes, like other anthropomorphic features found in nature, are quite intriguing and tend to spur the imagination. I think that aspen eyes have the blank, mysterious look of the ancient Egyptian Eye of Ra or the Eye of Providence symbol found on the US one dollar bill that has generated so many conspiracy theories.