Wood - used to heat more homes and for the construction of more family dwellings than any other material. That is why, from a global perspective, waste of this resource (Aggie Bonfire) is viewed as an act of ignorance. Forests cover ca. 1/5 of the earth's surface and constitute as estimated 90% of the earth's biomass. Global forest cover has decreased 19% since 1700 and crop lands have increase 466% (table 17-1). The full ecological dynamics of this human-mediated change in the world's flora are difficult to determine but atmospheric balances, especially CO₂, are now under consideration as an element of the 'global warming' problem.

Wood defined: tissue derived from vascular cambium - secondary (not derived from a primary meristem) xylem (mostly), ray cells, and phloem. Technically, secondary xylem is present in the vascular bundles of some herbs, derived from an interfasicular cambium, but wood, in the sense of a functional product, is xylem produced by a full vascular cambium.

Sapwood vs. Heartwood:- bark (periderm) is all tissues OUTSIDE the functional vascular cylinder and secondary xylem of woody plants - it, in essence, replaces the epidermis of herbs. This includes both functional (living) and dead phloem cells, the phellogen ('cork cambium' or secondary meristem), phellem [cork] produced by the phellogen to the outside and phelloderm [soft, parenchyma tissue] produced by the phellogen toward in inside.

Hardwoods = dicots with many cell types in xylem [ray,vessel elements, phloem, tracheids

Softwoods = gymnosperm wood = mostly tracheids

annual rings - different rate of cambial cell divisions [and cell sizes] as a function of the annual cycle and also variation between growing seasons - DENDROCHRONOLOGY