Morphology and orientation of cellulose in the vessels of vascular bundles of wheat straw were studied. The results indicated that cellulose acts as the framework in the vascular bundles and cellulose chains are high in orientation. In the thickening part of the vessels, cellulose exists in the form of cellulose crystalline lamellae but not cellulose microfibrils. The crystalline lamellae are perpendicular to the tangential direction of annular rings and incline clockwise with an angle of about 30-40 degrees to the tangential direction of the spiral line in the spiral vessels. A model of the arrangement of cellulose chains in the vascular bundles was proposed. (c) 2005 Elsevier Ltd. All rights reserved.