Hysteresis and Adsorbed Water in the Cell Wall

It is important to appreciate that water molecules do not penetrate a porous cell wall. Rather the dry cell wall expands as water is adsorbed within the non-crystalline regions of the wall and the accompanying volumetric swelling of the cell wall roughly corresponds to the volume of water adsorbed. Conversely when green wood is dried the volumetric shrinkage of the cell wall corresponds to the volume of adsorbed water that is removed. 

The amount of moisture adsorbed by the cell walls is directly related to the humidity of the surrounding air. Relative humidity or relative vapour pressure is defined as the ratio of the amount of water vapour in the atmosphere to the amount 

Any change in one of the variables (temperature in this case) that determines the state of a system in equilibrium (adsorbed water in wood in equilibrium with water vapour in the surrounding environment) causes a shift in the position of equilibrium in a direction that tends to counteract the change in the variable under consideration. 

Water vapour + wood adsorbed water in the wood + release of heat. (12) 

If fresh green wood is heated in water one would expect the amount of adsorbed water in the cell walls to deerease, the volume of the swollen cell walls to contract and the wood to shrink, even though it is still green. In practice the matter is more complicated due to the presenee of growth stresses in wood that are partially relieved at high temperatures (Yokota and Tarkow, 1962). 

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