Abstract:Cowpea stems can be right-handed helical when twinning on backers.We found that vascular bundles in skinless stems,which had being soaked in water,ranked lengthways as left-handed helixes,opposite to the stems’ twinning direction.In addition,helix and line type stem internode vessel elements were arranged as sectors when observed in crosswise transection by hand-held slicing method,and the angles of these sectors in helix type stems were significantly less than those in the same internodes of line type stems(p<0.05).The aspect ratios of all sectors were the same,except that in the first internode(p<0.05).These results show that the twinning helix and torsion helix counteracted each other,and the differences between sectors in helix and line type stems resulted from extrusiuoon producing in stem torsion.In addition,analysis showed that stem torsion was not the result of cell asymmetric transverse expansion in the xylem,but of unilateral cell elongation.