Variation of cross-sectional properties within single Norway spruce tracheids

The variation of cross-sectional tracheid properties along the tracheid length axis was studied in two Norway spruce (Picea abies) samples with varying cambium maturity. Twenty tracheids from both samples were measured for tracheid cross-sectional cell-wall area (A), perimeter (P), cell-wall thickness, and cross-sectional compactness (4πA/P2) at intervals of 5% of tracheid length using a confocal laser scanning microscope.Tracheid dimensions were largest in the middle parts of the tracheid. The rate of change was greatest in the vicinity of the tracheid tips. The cross-sectional compactness was quite invariant along the tracheid length axis. Tracheids were symmetrical with respect to their midpoint, and this symmetry was not affected by cambium maturity. The most representative location along the tracheid length axis for the mean of the whole tracheid appeared to be around 20% of tracheid length from tracheid tip. The results were consistent between the samples. On the average, the deviation from the mean varied from -60% (near tracheid tip) to +30% (middle of the tracheid), depending on the property in question. Naturally, the former deviation further increases towards the tracheid tips.Tracheid-to-tracheid correlations between the tracheid properties were not affected by the location of the measurement point within the tracheid length axis. The only exceptions were the locations near the tracheid tip, which often resulted in erroneous correlations.