Human: With regards to the structure of a leaf, how many cells are there in a leaf? Assistant: 
That depends on the leaf. Some leaves have more cells than others. A leaf is made up of two layers of cells. The outer layer is called the epidermis, and the inner layer is called the mesophyll. The epidermis is usually made up of two layers of cells. The bottom layer is called the palisade layer, and the top layer is called the spongy layer. The mesophyll is made up of two layers of cells, as well. The bottom layer is called the ground tissue, and the top layer is called the vascular tissue.