Comparative studies were carried out on the morphology and histology of 5 Clematis species, namely, C. intricata, C. aethusifolia, C. glauca, C. tangutica and C. hexapetala, allegedly used as "Tougucao". New characteristics, a sclerenchymatous ring composed of primary phloem fibers and lignified parenchyma cells, and a parenchymatous tertiary ring were discovered. A parameter representing the ratio of the diameter of cambium ring to the diameter of the stem (RCR) is created. A key for identification of the stems of the above five species is made according to the presence or absence of the sclerenchymatous ring, the diameter and numbers of hair cells, the thickness of fiber wall and the value of RCR, etc.