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During the whole time of the activity of the cambium ring and the formation of wood on its interior, it must not be forgotten that the outer rows of cambial cells are passing over into the tissue known as bast or secondary phloëm (also called secondary cortex); the chief differences in the process being (1) that much

Fig. 29.—A small piece of one annual ring of old oak wood (magnified twenty diameters): a, boundary of the autumn wood of the preceding (older) ring; b, that between the zone shown and the next youngest ring. In the annual ring shown the spring wood begins with large vessels, c and d, some with tyloses, d, in them, and passes gradually into autumn wood, with smaller vessels, e, e, and more tracheids and fibers, g. Only small medullary rays, i, are shown. (Hartig.)

less phloëm than xylem is formed; (2) that the elements do not become lignified; and (3) that the disturbances in the arrangement of the elements are more profound from the continued pressure exerted upon them between the resistant wood and the elastic periderm and bark, on the one hand, and the increased extension tangentially which it undergoes as the thicken-