Juvenile-type growth form

Wood is structurally composed of fiber shaped cells. The surface texture varies from species to species, hardwoods to softwoods. In addition to grain orientation that results from the fibrous nature of wood, different types of wood occur within a tree. Juvenile wood is formed during the first years of a tree s life and is composed of relatively short fibers. The heartwood of a tree differs chemically from the sapwood. Annual growth consists of alternating rings of springwood and summerwood, each differing in density and porosity. All these factors account for differences in texture as well as permeability, density, and compressibility. 

Other distinct classes of wood in a tree include the portion formed in the first 10—12 years of a tree s growth, ie, juvenile wood, and the reaction wood formed when a tree s growth is distorted by external forces. Juvenile fibers from softwoods are slightly shorter and the cell walls thinner than mature wood fibers. Reaction wood is of two types because the two classes of trees react differentiy to externally applied stresses. Tension wood forms in hardwoods and compression wood forms in softwoods. Compression wood forms on the side of the tree subjected to compression, eg, the underside of a leaning tmnk or branch. Tension wood forms on the upper or tension side. Whereas in compression wood, the tracheid cell wall is thickened until the lumen essentially disappears, in tension wood, tme fiber lumens are filled with a gel layer of hemiceUulose. 

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