Chapter 26: Secondary Growth in Stems

Secondary growth enables stems and roots to increase in diameter through the activity of lateral meristems, a process that distinguishes perennial woody plants from annuals that complete their life cycles in a single growing season. The vascular cambium, composed of fusiform initials and ray initials, drives this expansion through periclinal divisions that add secondary xylem inward and secondary phloem outward, while anticlinal divisions expand the cambium itself to maintain proportional growth between vertical and horizontal tissues. Seasonal cambial activity in temperate environments produces distinct growth rings that reflect annual environmental conditions, though tropical species often exhibit continuous cambial activity without visible rings. As secondary growth progresses, the primary body is substantially remodeled: primary phloem is compressed, and the cortex and epidermis are shed and replaced by the periderm, a protective tissue system consisting of the cork cambium, cork cells, and phelloderm. Cork cells, impregnated with suberin and often lignin, create a waterproof barrier essential for woody plant survival, while specialized lenticels permit gas exchange. The periderm is functionally divided into living inner bark composed of secondary phloem and dead outer bark consisting of successive periderm layers and senescent tissues. Wood structure varies significantly between conifers and angiosperms: conifer wood contains primarily tracheids and resin ducts, whereas angiosperm wood includes vessel elements, fibers, and diverse parenchyma cells. Vessel distribution patterns distinguish ring-porous woods, with large earlywood vessels characteristic of species like oak, from diffuse-porous woods with uniform vessel distribution. Functional specialization occurs within secondary xylem as heartwood develops from nonfunctional older xylem infiltrated with defensive compounds, providing structural durability, while sapwood remains active in water conduction and nutrient storage. Reaction wood develops in response to gravitational stress, appearing as compression wood on the lower side of conifer stems or tension wood on the upper side of angiosperm stems, each employing different anatomical mechanisms to restore vertical growth. Wood properties including density and specific gravity directly influence mechanical strength, hardness, and fuel value across diverse species.