As a plant grows, the stem often becomes thickened and somewhat woody since it is responsible for supporting the upper branches and leaves. cells in this area of the plant develop a. many mitochondria. b. extra chloroplasts. c. thickened cell walls. d. large central vacuoles.

Answer;

-Thickened Cell Walls

Explanation;

-As plants grow, stems and branches thicken or increase in girth. This thickening is a result of cells in the vascular cambium. Vascular cambium can develop into either xylem or phloem tissue within a plant.

-Woody plants have an extra layer of protection on top of the epidermis made of cork cells known as bark. The vascular tissue of the stem consists of the complex tissues xylem and phloem which carry water and nutrients up and down the length of the stem and are arranged in distinct strands called vascular bundles.