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Copyright (c) 2018 International Journal of Horticultural Science
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During the winter dormancy period of 1998-99, the differentiation process of xylem and the formation of annual and pseudo-annual growth rings was studied in sections of the central stem of Thuja orientalis of different age (1-14 year old), starting from the top (the tip of leader shoot) towards the bottom. In the apical 1-2 cm portion of the one year-old leader shoot, only the protoxylem was formed by the end of vegetation. The protoxylem elements appeared first in 6 bundles than gradually merged into 2 semiquadrangular (triangular) bands (each containing 3 protoxylem bundels) around the pith. In this stem portion, the pith was cross-formed first and became gradually flattened at the lower stem parts, following the generally flattened shape of the stem and the respective facial and marginal position of leaves. A continuous xylem ring (with formation of metaxylem elements) apeared 3-5 cm below the shoot tip. In fact, it was the stem part where a "real" annual ring was formed by the end of vegetation. The first pseudo-annual rings were observed 16 cm below the top. The "regular" annual rings were completely continuous all around the stem, consisted of strongly flattened in radial direction thick-walled latewood tracheids and had a distinct border (demarcation line) at the end of the year. The "pseudo-annual" rings formed incomplete dark rings or semicircular bands within the earlywood. They were composed of tracheids with thick cell walls but somewhat wider radial diameter than those of the "real" annual rings, and the border between their outside margin and the next earlywood was less distinct.
In the xylem of two year old stem portion, the innermost central annual ring appeared not at the transition zone between the current and the former years of growth, but about 2 cm lower. Above that, only the thick-walled bundles of the former years protoxylem were found. Down the stem, the older sections showed similar features: the next annual ring appeared always somewhat lower than the borderzone of the given and the former years growth. The "pseudoannual" rings (or more correctly the growth rings) continued regularly to appear in the lower (older) sections of the stem as well. They were found untill the age of up to 14 years (the bottom of the studied plants). Their number was 3-4 per year first, than (from the 5th annual ring counted from the centre) decreased to 3,2,1, and in the youngest outer part of the xylem there was no pseudo-annual ring at all. The development of pseudo-annual rings was usually more marked on the thicker (more branched) than on the thinner side of stem. Stems older than 14 years were not studied.