Mechanism of formation of spiral grain in Aesculus stems: dissymmetry of deformation of stems caused by cyclic torsion

The reversibility of deformation of young (3-10-year-old) <i>Aesculus hippocastanum</i> stems was investigated after subjecting them to alternate torsion to the right and left as well as the orientation of the microfibrillar helix in the main S₂ layer of the secondary cell wall in fibres. The studies demonstrated that residual torsions to the right are larger than to the left. The orientation of the fibrillar helix is always Z-wise. The dissymmetry of the mechanical properties of the stem may be responsible for the formation of Z (right-oriented) spiral grain in the stem of a tree undergoing torsions in various directions under the action of winds

Complex wavy grain in the stem of Aesculus

Two types of wavy grain were found in the xylem from <i>Aesculus</i> stems: with long (500 mm) and short (6 mm) waviness superposed on spiral grain. On radial splits the ridges of long waviness ran obliquely up the stem from the pith to the cambium on the average at a 5° angle with the stem axis. On the other hand, the short waviness ran down the stem from the pith to the cambium at an average angle of 86° with the stem axis. Investigation of the orientation of cambial events on the basis of splitting and uniting rays in the successive terminal xylem layers demonstrated that domain patterns in the cambium are responsible for the wavy grain.