What is the difference between an annual stem and a three-year stem?

If it is the stem of a herbaceous perennial plant, then there will be little difference. But still they will be. The stem grows larger over time. It receives an influx of various minerals. Organic matter stored in the stem expands it. This applies to all plants. If we take bushes or trees, then young trees have a green stem. Over time, it changes its color and becomes covered with bark. The more years a plant is, the larger its stem, and the more cracks in the bark form over time.