The gradient of water concentration that exists across the cortex creates a pushing force called "root pressure", i.e. a pressure that "pushes" the water across. Root pressure can be demonstrated by cutting a stem at soil level, after a time, droplets of water can be seen exuding from the cut surface. In some plants the process occurs naturally as "guttation", when droplets of water are forced through special pores (hydathodes) on the leaf edges. It is likely that the process of root pressure requires energy, as metabolic poisons halt the process.

While root pressure might account for some of the upward movement of water in small herbaceous plants, it is insufficient to overcome the force of gravity and move water up tall plants. To determine what else is involved in ensuring water gets to the sites of photosynthesis, let us look at the plant's water transporting tissue, the xylem.