Tension Wood and Eccentric Growth in Crack Willow (Salix fragilis, L.)

Shoots of S. fragilis in their first year of growth were bentthrough 360^° and grown in this position for 11 weeks. Duringthis period the shoots made greater radial growth on the upperside of both top and bottom halves of the loops. Coincidingwith this eccentric radial growth was the consistent productionof gelatinous fibres on the side with the longer radius, orin the region of the longer radius. Under these experimentalconditions both eccentric growth and tension wood productionwere related to the stimulus of gravity and not to tension orcompression forces. The quantitative results obtained clearlyshow a statistically significant degree of eccentric growthof the stem, and also a positive correlation between the maximumeccentricity and the localization of tension wood fibres inboth upper and lower halves of the loops. A 26-year-old horizontal branch of S. fragilis was also examined.It showed little overall eccentricity and no consistent eccentricityin the same growth ring on opposite sides of the branch. Tensionwood was present in the majority of growth rings on both upperand lower sides.