Abstract: Wood structure of three species i.e. Larix chinensis Beissn., Alnus sibirica Fisch. and Quercus mongolica Fisch. of normal and dwarf trees were comparatively studied. The age variation of width of annual rings and size of tracheary elements in these trees follows their own regularities respectively during the course of ontogenesis. In normal, as trees grow, the width of annual rings increases gradually, reaches maximum, then declines in subsequence; while the size of tracheary elements exhibits a crescendo increment, and remains constant after reaching maximum. These trends were not affected by the change of enviromental conditions in which the trees grow. However, the width of annual rings and size of tracheary elements of dwarf trees which grow in adverse circumstances are respectively narrower and smaller than those of normal trees. Further, in dwarf tree of L. chinensis (coniferous wood); the diameter of bordered pits on tracheids of early wood decreases, ray height is lower and ray frequence is higher. In dwarf tree of A. sibirica (diffuse porous wood), vessel frequence increases and the number of bars per perforation decreases. The quatitative features of wood structure of dwarf trees, as compared with those of normal trees, showed more variations; whereas, their own qualitative features of wood structure remain constant. These findings might indicate that modifications of wood structure responsing directly to change of enviromental conditions are usually only qualitative.

Key words: Normal tree, Dwarf tree, Wood, Annual ring, Tracheary element