Abstract: The leaf epidermises of 373 species (belonging to 204 genera) of the family Poaceae were examined under light microscope. It is considered that in Poaceae the conbinative patterns of constituent cells on the lower epidermis of leaf bl ade may be devided into five major types, i. e. Bambusoid type, Oryzoid type, Panicoid type, Chloridoid type and Pooid type; and the taxa differentiated with the characters of the five types are Bambusoideae, Oryzoideae, Panicoideae, Chloridoideae and Pooideae. This result is in ngreement with the five subfamilies devided by authors according to the morphological difference of constituent cell, also similar to some subfamilies confirmed by our predecessors according to the external morphology. In the evolution, some distribution characters on the lower surface may be the most primitive in the family, for example, the stomatal apparatus are gathered in 2-many rows on each side of the in tercostal zone, the microhairs are distributed between the veins, the papillae are distributed densely and the number on each long-cell is more, and so on; and others advanced or slightly advanced distribution characters, for instance, the stomatal apparatus are distributed in 1~2 rows on each side or in the middle part of intercostal zone,the microhairs are only distributed in the middle part of intercostal zone or are absent,the papillae are distributed sparsely and only one is borne on each long-cell or are absent, and so forth-On these grounds, the evolutionary level of the five types were discussed, and the relationships among them correspond to the external morphology and geographic distribution of taxa. The last results show that Bambusoideae is the most primitive in poaceae, Oryzoideae is slightly more advanced than the former, the evolutionary position of Panicoideae is intermediate in the family, Chloridoideae is more advanced than it, and Pooideae is the most advanced among the five subfamilies; the family Poaceae might originate from the World's tropical zone.

Key words: Poaceae, Leaf epidermis, Constituent cell, Combinative pattern, Evolutionary trend