In true OO development, each leaf is an object. Each branch is an object with a collection of objects (leaves). Each leaf has its own properties - validation prevents incorrect shaped leaves from being added to the collection (branch).

A leaf's parent is a branch. Each branches parent is the tree.

>OK, a non-evergreen (can't think of the word) tree has a temporary property or "leaf shape" because when it has no leaves it has no leave shape and you can't say that the leaves are seperate objects because they are part of the tree. The value of "Leaf shape" would be null for the all of the winter months so there would only really be a point in having the property in Summer.