The green balls represent blood stem cells in a stable "basin" on the landscape, where they remain stem cells. Each position on the landscape that the balls occupy corresponds to a gene expression state and can be assigned an "energy." An increase in the balls' energy or movement within the basin enhances the likelihood that a ball will escape from the basin, but does not bias it towards a particular fate (in this case, red or white blood cells). Only a change in the landscape induced by a differentiation factor may tip the balance toward another stable state, causing the stem cells to "roll down the valleys" and differentiate to either red or white blood cells.
Courtesy Sui Huang, MD, PhD, Children's Hospital Boston and University of Calgary