Cengage Horticulture Exam 1 Practice 2026 - Free Horticulture Practice Questions and Study Guide

Secondary growth and girth increase in trees and shrubs come from the vascular cambium, a lateral meristem that runs between the older xylem on the inside and phloem on the outside. As the cambium divides, it adds new secondary xylem inward and secondary phloem outward, steadily thickening the stem or root and forming wood over time. The epidermis is just the outer protective layer in young stems and is eventually replaced by the periderm; it doesn’t drive the widening. Xylem and phloem are the tissues produced by the cambium—products of secondary growth—rather than the tissue responsible for increasing girth itself.