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figure 7.30 Cross section of Pinus sp. early wood (E) and late wood (L) transition (Extant). Bar = 150 pm.

figure 7.30 Cross section of Pinus sp. early wood (E) and late wood (L) transition (Extant). Bar = 150 pm.

in this view or, in other words, at right angles to the ray's length. It is now possible to see the height and width (thickness) of the ray and the ray cells but not the length of the ray. To continue the brick wall analogy, in a tangential section you see the ray head on, like looking at the end of a brick wall, but you cannot determine its length.

Growth rings or tree rings occur in many woody plants (FIG. 7.30). Those that grow in temperate zones usually, but not always, produce a single ring each year and this can be counted to determine the age of the tree. Many tropical trees also produce rings, however, and they often correspond to wet and dry seasons. In some areas of the temperate zone, trees can produce multiple rings per year due to seasonal precipitation. Tree rings are made up of earlywood and late-wood, sometimes called spring wood and summer wood. In the spring, the apical meristems and young growing leaves of the plant produce the plant hormone, auxin, which is integral to the functioning of the vascular cambium. In the spring when the stems and roots are still elongating, auxin levels are higher, and the cambium produces earlywood—larger diameter tracheids with relatively thin walls. As elongation slows down and eventually stops, less auxin moves down the axis (or up in a root), and cambial production switches to producing latewood, which consists of smaller diameter

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