Secondary Growth in the Dicotyledonous Stem

A typical monocot stem, as the previous sections have described, is essentially fixed in width once its primary structure is complete, because it has no vascular cambium. A dicot stem (and a dicot root) is different: it retains two lateral meristems that stay active long after primary growth has finished, and their continued activity is what lets the stem thicken year after year -- a process called secondary growth.

The first of these lateral meristems is the vascular cambium. In a young dicot stem, a strip of cambium already exists within each vascular bundle, between its xylem and its phloem (this is called fascicular cambium, since it lies inside the "fascicle," or bundle). As the stem matures, the parenchyma cells of the medullary rays lying between adjacent bundles also become meristematic, forming interfascicular cambium; the fascicular and interfascicular cambium together join up into one continuous ring of vascular cambium running all the way around the stem. This cambium ring then divides repeatedly on both of its faces: toward the inside it cuts off new cells that mature into secondary xylem, and toward the outside it cuts off new cells that mature into secondary phloem. Over time the cambium typically produces much more secondary xylem than secondary phloem (partly because the older secondary phloem gets crushed and pushed outward as more xylem accumulates within), which is why the bulk of a tree trunk, when cut across, turns out to be built almost entirely of accumulated secondary xylem -- what is commonly called wood.

In a region with a distinct favourable growing season (such as spring) followed by a less favourable one (such as late summer or autumn), the vascular cambium's activity is not constant through the year, and this unevenness leaves a visible record in the wood. During the favourable season the cambium is highly active and produces xylem vessels with wide lumina and comparatively thin walls -- a lighter-coloured, less dense band called spring wood (or early wood). As conditions become less favourable, cambial activity slows, and the vessels it produces are narrower, with somewhat thicker walls -- a darker, denser band called autumn wood (or late wood). One band of spring wood followed by one band of autumn wood together make up a single annual ring, and because one such ring is normally laid down in each growing year, counting the annual rings visible in a transverse section of an old woody stem gives a fair estimate of the number of years the stem has been growing -- a technique used, among other things, to estimate the age of trees. …