When dry wood pieces and seeds of certain tree species are put in water, they absorb water and swell noticeably. This phenomenon of water absorption is called imbibitions.
Imbibition is due to the presence of large number of hydrophilic colloids in both the living and dead cells of wooden materials. They include proteins and carbohydrates such as starch, cellulose and pectin. In the case of dry wood, there are more dead cells with lignified walls. As a result it readily absorbs and releases water depending on the environmental conditions.
For example, during rains, relative humidity of atmospheric air is high. Hence, wooden doors absorb water molecules and swell. If the clearance between the door frame and door is insufficient to accommodate the swell, we experience difficulty in closing and opening it.
Periodic painting helps to reduce moisture absorption and the difficulty.