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Lead acetate precipitation method was used for ultracytochemical localization of aloin. The processes of aloin production, transport and storage were studied by transmission electron microscope. Results showed that aloin was produced in the plastids of the assimilating tissue. The aloin was transported through the plastid membrane to the surrounding endoplasmic reticulum and enveloped in the vesicles by the endoplasmic reticulum elements, the vesicles approached and later fused with the plasmalemma. Some vesicles of the plastid membrane directly fused with the plasmalemma. The vesicles released their contents into the apoplast through exocytosis, and finally reached the vascular bundle sheath by apoplastic translocation. Aloin was transported to the internal tangential wall of vascular bundle sheath cell through endoplasmic reticulum vesicles, and reached the cytoplasm of aloin cell by means of plasmodesmata. Finally, aloin was stored in the vacuoles of aloin cell.