Abstract: We analyzed the main character differences of seventy-two reprehensive populations from twenty four Leymus species previously reported by scholars by morphological observation and anatomical analysis of leaf epidermises. There are different degrees of difference in fifteen morphological characters including length and width of flag leaves, hairs of leaf blades, straight/curve and length/width of inflorescence, spikelet No. of each inflorescence, floret No. of each spikelet, length of lemma and palea. The tested species can be divided into three sections according to them. The sect. Racemosus species of L.racemosus, L.crassiusculus, L.pluriflorus and L.pseudoracemosus, with sturdy, erect and thick spike, 3-number of spikelets per rachis node, 3-10 florets each spikelet, linear lanceolate glume, etc., is the most primitive one in genus Leymus. Due to the sect. Anisopyrum species including L.pishanica, L.golmudensis and L.ruoqiangensis, having thin, erect and loose spike, only one spikelet per rachis node, usually 2-5 florets each spikelet, linear lanceolate or cone glume, etc., so it is the most advanced of them. Meanwhile, because the external morphological characters of the sect. Leymus species usually lie in the intermediate transition state between the first two groups, naturally its systematics position should also be between the sect. Racemosus and sect. Anisopyrum species. At the same time, based on the evolutionary trends of the external morphological characters of three above sections, we deduced that the sect. Racemosus might produce immediately the sect. Leymus, whereas the sect. Anisopyrum might derive immediately form the sect. Leymus. The leaf epidermis of Leymus from China consists of long cell, stomatal cell, short cell and prickle hair. It belongs to the typical festucoid type. Except that some characters, such as the type of long-cells and thick/thin of cell walls, the volume and type of guard cells from stomatal apparatus, have obvious overlap and show similar, other characters, including the length of long-cells and curve extent of cell walls, distribution pattern of short-cells, shape of subsidiary cells, and type of prickle-hairs, have obvious differences. They can delimitate Leymus species into three sections identical with the morphology. According to the evolutionary trends of leaf epidermal characteristics of three sections and species within each section, their evolutionary positions and relationships were inferred. The results show that the sect. Racemosus is the most primitive one among three sections, the sect. Leymus is slightly more advanced than the former, and the sect. Anisopyrum is the most advanced of them. The sect. Racemosus might produce immediately the sect. Leymus, whereas the sect. Anisopyrum might derive immediately form the sect. Leymus. The biosystematic relationships of taxa below the genus are corroborated by the evolutionary trend of external morphology.

Key words: Poaceae, Leymus, morphological character, morphological of leaf epidermis, biosystematic relationship