Abstract:This paper deals with the haustorium development of parasitic plant (Cassytha filiformis L.) parasitized on purple willow (Salix purpurea L.).The polarity occurred in the portion near the host; and the cushion-shaped haustorial plate formed, and then the haustorial primordium initiated in the cortex. Finally, the haustoria reached the cortex of pith of the host, penetrating through its own cortex and epidermis. Tracheary elements were differentiated from the base of the haustoria and the sieve elements were not observed in the haustoria. Histochemical studies revealed that there were starch grains in normal stem cortex. The starch grains were increased in the portion near the axis after twisting on host. After the haustorial plate was formed, the starch grains were richly accumulated in the central group of cells, which were followed by the haustorial development; The starch disappeared in the meristem, in which protein stained deeply; The dynamic change of protein turned oK to be contrary to the tendency of starch accumulation. The structure and parasitic mechanism and the dynamic change of starch and protein are discussed.