One of the most frequently encountered arrangements in trees is pairs of leaves and branches that emerge exactly opposite one another. After the seed sprouts, it opens up two rudimentary leaves, which leaves are set out 180 degrees opposite one another. The two leaves that sprout above these first two also grow opposite one another and at right angles to the first pair, for the greatest possible distribution between them. In this way, there are now four leaves spaced every 90 degrees on the stalk. In other words, if we look at this branch from above, we see that the leaves are so arranged as to constitute a square, and that the upper leaves thus do not shade the lower two.28