How many leaves are on a tree? The key may be that any leave -- whether or not it is on the surface of the tree or inside of it -- will die unless a certain percentage of its area is exposed to the sun. For example, if the tree (without the trunk) is a ball of radius r = 3 m, then its projection on any plane -- and hence on any plane perpendicular to the sun beam -- is of area A = πr2, approximately 30 m2 = 300,000 cm2. If the area of one leaf is a = 10 cm2, then by this logic there are approximately A/a = 30,000 leaves on the tree, or something of this order of magnitude.