Woods from different species of tree or
bush have different properties because of the makeup of the cells
inside. The density of wood depends on how much of each kind of
material is in the wood. One component that affects the density very
much is the amount of air in the wood.
Tree trunks and bush
branches have two main functions (and many others, too!) -- supporting
the plant and providing nutrients, particularly water, from the soil.
(They also offer protection, fire and insect resistance, a place to put
chlorophyll during the long winters, and lots of other things). To
transport water, wood has many small tubes which draw water through
them by capillary action. See our discussion on capillary action
for more details.
wood is inside a living tree, these tubes are full of fluid (water and
tree sap). When the wood is dried, the tubes are left with air and it
is much lighter. So the density of wood depends quite a bit on how dry
it is, but it also depends on how many and how big those tubes are. The
remainder of the material in the cells (cellulose and other
constituents) also varies in composition from tree to tree depending on
the needs of the tree -- tall trees with thin trunks may need stiffer
support, while short trees may be different. Some trees, like
eucalyptus trees, have a lot of oils stored in the wood, and oils are
less dense than water.
(republished on 07/18/06)