Final answer:
In a stem cross section, the abrupt change between small summerwood cells and the following year's large springwood cells produces annual rings.
Step-by-step explanation:
In a stem cross section, the abrupt change between small summerwood cells and the following year's large springwood cells produces a characteristic pattern known as annual rings. Annual rings are parallel lines observed in the cross section of wood and are formed due to the alternation of earlywood (springwood) and latewood (autumnwood) during the tree's growth. By examining the number and characteristics of the annual rings, such as size and cell wall thickness, the age of the tree and the prevailing climatic conditions during each season can be revealed.