Androecium Angiosperm Animal dispersal Animalia Anise Annelid Worms Annual Rings Annual rings are the layers of wood laid down by the vascular cambium in a woody dicot or a gymnosperm. Because the vascular cambium is active in the spring through the fall, xylem cells are being added to the circumference of the xylem tissue. In the spring these cells grow fast and are large, however as the summer progresses into fall, the numbers of cells being produced slows down and finally stops. At the same time, the cells get smaller and smaller. The next spring when the vascular cambium breaks dormancy, the cells are large and fast-growing again. Thus, the difference in size between the small cells ( summer wood) and the new spring cells ( spring wood) forms a line between the spring and summer wood. This then marks the boundary of an annual ring. DIAGRAMS: PHOTOS: Annual Rings