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In large trees the seasonal fluctuation will affect the growth of secondary xylem. Two types of wood formation occur during the spring season and the summer season they are called as the spring wood and the summer wood respectively. The xylem formed during the spring season contains numerous large vessels with wider lumen and possess less amount fibres. So it acquires a greater radial width than the summer wood. The xylem formed during the summer and autumn season is very less amount since the activity of the cambium is highly reduced. The vessels are thick walled with narrow lumen and the summer woods possess mainly fibres and narrow vessels.

The spring wood is also called as the early wood whereas the summer wood or autumn wood is called as the late wood. The early wood and the late wood occur appears in the form of concentric rings known as the annual rings.

Image Source: http://www.plantstogrow.com/Botany/fact_pages/images_3/annual_rings-02cm.jpg

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