Graduate Thesis Or Dissertation
 

Seasonal pattern of secondary growth in stems of northern white spruce (Picea glauca (Moench) Voss)

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  • Cambial activity in white spruce stems in Alaska was observed from April, 1964 to September, 1965 in over 300 samples taken from 50- to 60-year-old natural stands. Mitotic index was used as a measure of the rate of periclinal division of fusiform cells in the cambial zone. Anticlinal divisions are relatively rare, occurring only about once per 278 periclinal divisions in most stems. Mitotic index, at any given time, is quite uniform throughout the cambial zone of an internode, among internodes of the same tree, and even among trees of a stand growing at markedly different rates. There may be some difference in rate of division across the radial extent of the cambial zone.. During dormancy, the radial number of fusiform cells in the cambial zone (NCZ) ranges from two to about eight, the number being related to annual growth rate of xylem and phloem. Two to three overwintering immature (precocious) sieve cells border the cambial zone in each radial file of cells. They are the last phloem derivatives of the previous year's cambial activity. They undergo rapid radial enlargement just prior to vernal reactivation of mitosis in the cambial zone, and they become the first new conducting phloem elements of the current year. There are three distinct growing season periods: early period, grand period, and late period. Beginning of early period cambial activity is marked by reactivation of periclinal divisions, first near the apex, then at successively lower stem levels. Each cambial zone fusiform initial divides once before any new xylem or phloem derivatives are produced, thus approximately doubling the NCZ. Mitotic activity varies considerably during the early period, and the time required to double the NCZ varies from year to year depending upon weather conditions. Production of the first new xylem and phloem derivatives marks the beginning of the grand period. Rate of cell production in the cambial zone remains about equal to derivative production for the next 45 to 50 days, when about 80 percent of annual xylem and phloem increment is produced. Rate of division is comparatively uniform in all internodes in all trees. Rate of derivative production is, therefore, dependent on NCZ. There is a rather abrupt drop in NCZ at the beginning of the late period, apparently due to lengthening of the cell division cycle. Rate of derivative production exceeds rate of cambial zone cell production, and NCZ soon drops to about the dormant level. Subsequently, within about ten days, derivative production slows, the zone of developing tracheids begins to narrow, and radial enlargement of developing tracheids and sieve cells declines. In about two more weeks the zone of developing tracheids is almost eliminated, the most recently produced tracheids show little or no radial and there is evidence of partial inception of dormancy in the cambial zone. Complete termination of cambial activity is gradual, extending through late August and, perhaps, into September.
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