Roots of Purshia tridentata show eccentric growth

Abstract

Many publications have described eccentric growth (non-uniform growth of secondary xylem) caused by the death of the vascular cambium. The purpose of the present research was to determine if roots of Purshia tridentata (Pursh) DC exhibited eccentric growth. Eccentric growth in roots of P. tridentata was studied with three sequential segments 8 mm apart for 5 roots. Data of the three sequential segments provided a method to determine changes in eccentric growth along roots. Each wood segment was subdivided into ten sectors of 36° each. The areas of xylem rings within each sector from year to year within each segment were used to determine vascular cambium activity. For more than 95% of all rings, areas of at least one sector were significantly different from areas of other sectors for all roots. Except for one first- and second year xylem rings, all other xylem sectors were eccentric. Among sequential 8 mm root segments more than one-half of segment pairs were statistically different. The above results demonstrate that root eccentricities were very localized both within individual segments (sectors within each segment) and along roots (from one segment to the next segment). In summary, the five root samples were eccentric and showed very similar ring patterns, growth rates and eccentricity patterns. To our knowledge this is the first publication to show eccentric growth of roots similar to eccentric growth of stems.