Each cell is simulated independently, and fluid flow of nutrients is simulated between cells as well as between the plant and the environment. The plant needs to nutrients in order to grow. Growth decisions like when to divide cells and where are influenced by the growth hormone auxin, which is diffused from cell to cell. Right now the simulation runs by itself given a set of initial parameters, but in the future these decisions can be influenced by the player.
Day/night cycles are simulated, and these affect photosynthesis rate.
I noticed that most growth decisions made by plants are really just resource decisions. This made think of real time strategy games, where players have to choose between "macro" investments in resource production, or "micro" investments in short-term armies.
I hope that plant rpg will be an educational tool for those interested in botany.