bio

Secondary growth

A stem thickening over time rather than only lengthening. In plants with a vascular cambium, a cylindrical layer of dividing cells adds wood inward and bark outward, year after year — that is what makes a woody trunk and its growth rings. The point that matters for classification: not every tall plant does it this way. The giant clubmosses of the Carboniferous reached forty metres with under a tenth of their diameter in wood, supported mostly by thick bark, and palms have no vascular cambium at all. Same tree shape, unrelated mechanisms.