Abstract: By means of scanning electron microscope (SEM),The morphological structure of the inner wall of the vessel molecules in plants of Betulaceae in Heilongjiang Province is studied,which includes 4 genera (Alnus,Betula,Carpinus and Corylus),15 species,4 varieties and 3 forms are included in the study.The result shows that there is obvious difference in the length and the width of vessel molecules.Pits are bordered.They are all alternate pitting except a few species.The pits between vessels are distributed in groups,most of them show regular form.In the inner or on the border of pits,there are round point projections or reticular projections that are consist of varying apertures.They are common in Betulaceae.The main features of the inner walls are discussed and analyzed the ecological adaptation in morphological evolution of the vessel molecules are analyzed.This paper concludes that the specialization level of vessel molecules is determined by the humidity utilization,traspiration and pressure selection.

Key words: Betulaceae, pit, bordered pit, alternate pitting, SEM