Abstract: Zosterophyllum yunnanicum, an important and widely distributedspecies in the Lower Devonian of southern China, was first described by Hsü in 1966.Later then, Li and Cai (1977), Cai and Schweitzer (1983) described the details of itsmorpholegy, and compered it with other members of the genus Zosterophyllum. In thispaper, the materials of its tracheids, epidermal ceils, stomata and its spores have beenstudied. The tracheids of Z. Yunnanicum are annular, and 9–30 gm across. Epidermal cellsof the axes are elongate, polygonaI, ranging from 70–280 μm by 12–60 μm. The stomata are mainly fusiform, 65–80 μm in length and 40–70 μm in width, probably consisting of a pair of guard-cells enclosing a pore, 8–20μm long and about 1.5μm wide. The density of stomata evidently varied in different parts of the plant. The portion of the sporangium near the rim is composed of rectangle and elongate cells. Theremainder wall of the valve consists of elongate cells, folding often. Spores from the sporangia are 20–55 gm (average 37.1μm) in diameter, smooth with thickened in area of trilete mark, which are 1/4–2/3 of the radius of spore. Some spores look similar to the dispersed spore Retusotriles (Naum.) Streel 1964. Apparently this Plant is homosporous.

Key words: Zosherophyllum yunnanicum, Stomatal apparatus, In situ spore, Tracheids