Annual growth rings
The rate of wood growth increases in summer and decreases in winter, producing a characteristic ring for each year of growth. Seasonal changes in weather patterns can also affect the growth rate. Note how the rings vary in thickness.
Source
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"OpenStax College, Stems. October 17, 2013."
http://cnx.org/content/m44702/latest/Figure_30_02_09.jpg
OpenStax CNX
CC BY 3.0.
