Abstract:Lots of small developed squamules 1—2 celled layers in thickness and without vascular bundle were observed in the leaf axils of Sagittaria sagittifolia. They were formed after divisions of epidermal cells at the basal portion of both dorsal and ventral sides of leaf sheath and leaf axils, and their origin bore no direct relation to the apical meristem. The apical meristem was covered up by such squamules after their development. Owing to the mucous secretory function of squamules, the apical meristem and leaf primordia were embedded in the mucilage which effectively served to protect them against attacks of water current. Cellular structure of the squamule in its active state of secretion showed under TEM the following properties viz. :number of dictyosomes increased significantly with vesicles budded off from the cisternae; the endoplasmic reticulum dilated to form the large vesicles; mitochondria distributed by the side of plasmalemma with deformed ones might be discovered. At the later stage of squamule development, vacuolation of cells occurred, all the cell organelles other than nucleus disappeared completely. The ultrastructural changes appeared in cells of squamule during process of development indicated that the cells of squamule possessed the characteristics of typical secretory function.