Introduction to Secondary Growth

A plant organ's growth begins at its apical meristems - the growing tips of root and shoot - which drive an increase in length, called primary or longitudinal growth. But that is only half the story for many plants: gymnosperms, most angiosperms and even a handful of monocots (notably tree-like palms and bamboos, and monocots such as Dracaena and Yucca) also grow outward, adding girth to their stems and roots over time. This lateral, girth-increasing growth is secondary growth, and it is the reason a grass stem stays thin and soft throughout its life while a neem stem becomes thick, hard and woody as the plant ages - grasses and most monocots lack the machinery for it, while dicots and gymnosperms have it built in. That machinery consists of two lateral meristems working side by side: the vascular cambium, which lays down new secondary xylem (wood) and secondary phloem (bast) to thicken the plant's conducting core, and the cork cambium (phellogen), which builds a new protective corky covering (the periderm) to replace the epidermis once it can no longer stretch to cover the widening stem or root. Everything covered in this chapter - the cambium's origin and activity, the wood and bark it produces, annual rings, anomalous patterns of growth, and timber itself - follows from the coordinated work of these two lateral meristems.