"Yes, our tree has an interesting shape. The center branches reflect the shape of the zero curve. When extreme parts of the tree are reached the branching pattern changes to accommodate the mean reversion"

John C. Hull's quote is describing the shape of a tree and how it associates with the idea of mean reversion. The "absolutely no curve" is a mathematical idea that describes the average rate of return of a financial investment over a particular time period. The center branches of the tree reflect this absolutely no curve, suggesting that the average rate of return is consistent throughout the tree. Nevertheless, when the extreme parts of the tree are reached, the branching pattern changes to accommodate the mean reversion. This implies that the rate of return is no longer constant, however instead changes in order to revert back to the average rate of return. This quote is an excellent example of how mathematical ideas can be applied to real-world situations.