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Why do woody plants form annual rings?

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fern life cycle — Plant development The life cycle of the fern. (1) Clusters (sori) of sporangia (spore cases) grow on the undersurface of mature fern leaves. (2) Released from its spore case, the haploid spore is carried to the ground, where it germinates into a tiny, usually heart-shaped, gametophyte (gamete-producing structure), anchored to the ground by rhizoids (rootlike projections). (3) Under moist conditions, mature sperm are released from the antheridia and swim to the egg-producing archegonia that have formed on the gametophyte's lower surface. (4) When fertilization occurs, a zygote forms and develops into an embryo within the archegonium. (5) The embryo eventually grows larger than the gametophyte and becomes a sporophyte.

Woody plants form growth rings due to the seasonal activity of the vascular cambium, a lateral meristem that produces secondary xylem (wood).12

These rings are visible because of structural differences between earlywood, which is produced at the start of the growing season, and latewood, which is produced later. Earlywood cells are typically larger with thinner walls to facilitate rapid water conduction, while latewood cells are denser with thicker walls. The sharp contrast between the dense latewood of one season and the large-celled earlywood of the next creates the distinct boundary of a ring.132

While historically called "annual rings," these structures are more accurately termed growth rings. In temperate regions, they generally form once per year in response to seasonal cycles. However, they can also form in response to other cyclic environmental conditions, such as fluctuations in water availability, or fail to form entirely during adverse conditions.12