Unusual growth patterns in short shoots of Pinus radiata

Short shoots and the needles that they bear usually persist for about 3 years on stems and branches of Pinus radiata D. Don. As the main axis thickens with secondary growth, short shoots may elongate by four times their original length, during which process xylem elements become embedded in the stem as "needle trances," although the external shoots appear to remain unchanged. This report describes how elongation in both xylem and phloem results from the insertion of new vascular elements derived from marginal cells of radial parenchyma bands. There was no evidence that the apical meristem contributes to this growth. Possible mechanisms for elongation of pith and cortex are also described.