Differentiation of Tracheids in Developing Secondary Xylem of <i>Tsuga Canadensis</i> L. Carr. Changes in Morphology and Cell-Wall Structure
Keywords:<i>Tsuga canadensis</i>, xylem, cambium, cell size, cell walls
The morphology and the changes in total cell-wall mass in developing secondary xylem of two eastern hemlock trees were studied. Sixty-μm-thick tangential-longitudinal sections were microtomed sequentially from the cambium through the currently developing and the one-year-old increments. The weight and volume of these sequential sections gave data on the rate of mass production. Cross-sectional microtome sections were used to study cell-wall structures and to measure cell-wall layer areas.
Four tracheid maturation zones could be measured and described during both early- and latewood formations. The size of the cambium, cell enlargement zone, and the zone of S1 cellulose framework formation were the same throughout the growing season. However, the size of the zone of S2 cellulose formation changed. This zone was only one-third as wide during the formation of the thick S2 layer in latewood tracheids as it was during the formation of the thin S2 layer in earlywood tracheids. Despite the fact that the number of cells produced during S2 layer formation in latewood was only one-third as many as in the earlywood zone, the rate of total mass production was more than twice as great compared to earlywood. Tracheid diameters and cell-wall layer volumes across both the currently developing and the one-year-old xylem showed that size development is complete for each layer before the appearance of the next inner layer in the tracheids. However, cell-wall layer densities continued to increase perhaps well into the second and subsequent growing seasons. Change in the relative proportion of the cell-wall layers across the growth increment was not dominated by the S2 layer. This relative variation of the S2 layer was the smallest of any secondary cell-wall layer across the growth increment. However, it constituted 50-70% of the total cell-wall volume.
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