In woody plants the new growth is herbaceous and becomes woody with age.
The development of woody tissues is seen in trees and shrubs. As the new cells produced by the cambium differentiate the rapid formation of xylem pushes out the encircling tissues and the stem increases in thickness. As the new xylem differentiates, its character is determined to a great extent by the amount of water available. The phloem is replaced each year with the previous years’ cells being replaced by new ones. The old phloem cells are recycled by the plant. The xylem cells however are harder and have strengthening fibres and they remain to add support for the plant. Each year a new layer of xylem tissue is produced adding to the girth of the stem.
Formation of annual rings and medullary rays
In spring, when the tree is full of water, many of the new xylem cells expand vigorously giving wide vessels and crushing the adjoining tissues so that fibres are formed. As the leaves open, less water is available to expand the differentiating vessels and so the xylems produced in the summer are narrower. Some of the cambial derivatives differentiate into medullary rays and these rays are common to xylem and phloem. They channels for diffusion of gases and water vapour.
Medullary rays = A parenchyma plate that extends between the medulla and the cortex across the vascular region, originating in the vascular cambium and running at right angles to the stem. Medullary rays transport water and nutrients across the stem.
As the leaves become fully expanded the cambial activity slows down and by July it has usually stopped though differentiation of the new tissues may go on for some time. The formation of further tissues is not usual, and so by the time that all the wood is fully developed and lignified, there is a well defined zone of wood representing the seasons’ growth.
This is called an annual ring.
These rings are produced each season and by counting the number of rings we can determine the age of a tree, since each ring represents one years growth.
Heart and sap
As growth continues the softer phloem vessels are squashed as more numerous and harder xylem vessels occupy and represent more of the cross section of the stem. The majority of the stem is occupied by secondary xylem which forms wood.
Illustrations by Hanna Huuri (© Copyright The Garden School -used with permission).Text and images by The Garden Scool- © Copyright The Garden School- reproduction prohibited)