Q1.
What causes this increase in girth?
Answer
A ring of actively dividing cells inside the stem — the lateral meristem.
These cells are arranged in a cylinder running along the length of the stem. When they divide, the new cells are added inside and outside the ring in a concentric manner, so the stem thickens from within while keeping its round shape.
Why it shows up as rings: the meristem divides fast in a favourable year (plenty of water, warmth) and slowly in an unfavourable one. Fast growth gives a wide band of large, thin-walled cells; slow growth gives a narrow band of small, thick-walled cells. One wide band + one narrow band = one annual growth ring (Fig. 3.4). Counting the rings gives the age of the tree, and their widths are a record of the climate the tree lived through.
Did you know? This is why dicot stems get thicker every year but a grass or a palm stem does not — they have no lateral meristem.