Capillarity works because water molecules are attracted to charged surfaces. The smaller the bore of the tube, and the greater the charge on its wall, the higher the water will move. Xylem vessels are very narrow bore tubes with highly charged surfaces (cellulose is highly charged). For mosses, this is fine, but the process can only account for a short height of water movement. Root pressure. We have already seen that water potential can move water to great heights. The water seen on leaves in the morning (morning dew) is supposed to occur by water taken up as a result of active ion uptake at night. During the night, the plant keeps its stomata closed so there is little transpiration, and the water osmotically taken up as a result of ion uptake exceeds traspiration and is released onto the surface of the leaves in a process called guttation. There are two main problems with this mechanism. The first is that the solutions get diluted as they take up water. This requires enormous energy inputs to maintain a concentrated salt solution in the xylem. To take up the volumes of water lost by plants, it has been calculated that the roots would have to produce so much energy that the water would boil. The second problem is even more serious. These salts cannot be recycled and would just accumulate in the leaves as the water was transpired causing immense osmotic and toxicity problems. Transpiration pull. Even in Ireland, the atmosphere usually contains a lower concentration of water than the plant. Thus is has a much more negative water potential. This causes a loss of water from the leaves which is transmitted through the plant to the root.
Just as in our original example of water potential, the water moves from the intercellular spaces to the atmosphere. Water is lost from cells to the intercellular spaces, and these cells then being at a lower (more negative) water potential than their neighbours will abstract water from them. This continues across the leaf until it reaches the bundle sheath cells. These abstract water from the xylem. Water is highly cohesive. The water molecules are bonded to each other by hydrogen bonds, and are thus very cohesive. Experiments to demonstrate the cohesive strength of water have generally put the strength of bonding of water molecules at several hundred atmospheres. (This only works in the absence of contaminating gasses which will come out of solution under vacuum, hence columns of water do get broken in the plant under normal physiological conditions). The entire column of water therefore is dragged up, and the reduction in pressure potential in roots literally sucks water in. Again this is propagated across the pith of the root up to the endodermis.
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