The strength of a given species however, is dependent upon cell thickness. The composition of wood is approximately 60 percent cellulose, 23 percent lignin and minor quantities of various other materials. The framework of the cell wall is formed of cellulose with lignin being the cementing agent that stiffens and binds the cell walls together. HARDWOODS AND SOFTWOODS - These general classes are used to divide all species into two groups. However, the names "hard" and "soft" do not indicate any definite degree of hardness. Actually some "hardwoods" (e.g. basswood and cottonwood) are softer that the average "softwoods". Some "softwoods" (e.g. Longleaf Southern Pine) are as hard as the average "hardwood". In general, most hardwoods are deciduous trees (cast their leaves in the Fall) having broad leaves. The softwoods are usually evergreen and have needle like or narrow leaves. GROWTH RINGS - As shown in Fig. 1, approximately concentric rings start at the pith and continue to the bark. For normal conditions each set of rings (dark and light) represents one year of growth. All growth originates at the cambian layer thereby pushing the bark out of each new layer is added. It is significant to note that the age of a tree can be predicted approximately by counting the number of annual rings of a cross-section taken through the trunk. This is with the provision that each year's growth is interrupted by cold or dry seasons. Obviously, annual rings are not a characteristic of trees grown in tropic or sub-tropic regions. SPRINGWOOD AND SUMMERWOOD - Most species of wood (having the growth ring characteristic) have annual rings composed of two distinct layers. One is made up of cells (fibers) having large cavities and relatively thin walls (springwood) while the other consists of smaller cells having thicker walls (summerwood). In most species, the summerwood is by far heavier, harder and stronger that the springwood. DENSITY (SPECIFIC GRAVITY) - The density (relative) of wood greatly affects the strength of wood. The higher the density, the greater the strength. The relative density includes in its determination the volume of cell cavities. It should be noted that the absolute specific gravity of all wood is approximately 1.54 regardless of species.