Is there any math formula that can be used to describe shape of leaves?
There are algorithms which can be used to describe leaves, if that is what you mean. 

The following description takes into account that leaf shape is not solely a function of genetics, but is also effected by environment. It also models how different growth rates within the cells have consequences. Mutant, cloning cells are also factored into the description.

The mathematical model has interesting allometric implications.

Here is a very readable paper (PDF),which endeavours to describe differences using mathematics and for which for which we can thank Cornell University Library for hosting the paper and Lawrence Livermore National Laboratory.