Abstract:The transverse sections of leaf blades of the major species of sect.Elymus,sect.Turczaninovia and sect.Macrolepis in Elymus were examined.The results showed that:(1)The leaf blade consists of epidermis,mesophyll and vascular bundle,which is isobilateral leaf.It belongs to the typical festucoid type.Namely,shape,magnitude and arrangement of epidermises are asymmetric.The mesophyll is not distinguished into palisade tissue and spongy tissue.They have double layer bundle sheath.The ambient mesophyll cells arranged irregularly and sclerenchyma is joined to epidermis.There are also obvious differences existing on the transverse section characteristics of leaf blades including shape,magnitude and depth of furrows in upper epidermises,number of large vessels,etc.(2)According to the evolutionary trends of transverse section characteristics of three sects,their evolutionary positions and relationships were inferred.The results show that the sect.Elymus is the most primitive one among the three sects,the sect.Turczaninovia is slightly more advanced than the former,and the sect.Macrolepis is the most advanced of them.The sect.Elymus might produce immediately the sect.Turczaninovia,whereas the sect.Macrolepis might derive immediately form the sect.Turczaninovia.The biosystematic relationships of these three sects are corroborated by the evolutionary trend of external morphology.