Growth rings form from the seasonal contrast between thin-walled springwood and thick-walled summerwood cells.

Evidence

Trees are affected by their sapling days as much as people are by their childhoods. The history of a tree appears in its growth rings, of course. Good years yield a wide ring, poor years a thin one, and the pattern of rings is critical to the process of basket making. … The cells of the cambium are perpetually embryonic, always dividing to add to the girth of the tree. In the spring, when the buds detect the lengthening of the days and the sap starts to rise, the cambium grows cells made for feast days, big, wide-mouthed tubes to carry the abundant water leafward. These lines of large vessels are what you count to determine a tree’s age. They grow quickly and so their walls tend to be thin. Wood scientists call this part of the annual ring springwood or early wood. When spring turns to summer, nutrients and water become scarce and the cambium produces smaller, thicker cells for leaner times. These densely packed cells are called late wood or summerwood. When the days shorten and leaves fall, the cambium settles in for a winter’s rest and stops dividing altogether. But as soon as spring is imminent, the cambium once again bursts into action, making large springwood cells. The abrupt transition between the last year’s small-celled late wood and the early wood of spring creates the appearance of a line, a growth ring. — Robin Wall Kimmerer, Braiding Sweetgrass, 501

Categorization

• DEFINE: Clear mechanism for how springwood/summerwood contrast produces the visible growth ring line — precise anatomical explanation bridging craft and botany.