Here is the principle for new cell selection.

Each new cell produces a revenue for the leaf, consisting of the solar energy which it collects.

This revenue is proportional to the area of the cell. The proportionality constant is given by a parameter \$$\epsilon\$$. Recall that all cells are squares of width \$$h\$$. Hence the revenue from any cell is given by \$$\epsilon h^2\$$.

On the other hand, there is the added cost of transporting fluid from the root to the new cell.

Principle: a cell just outside of the fringe of the current leaf will be added if its revenue would exceed its cost.