The hook of Uncaria rhynchophylla is an important external phenotype representing the quality of Gou-Teng. The hook formation pattern of U. rhynchophylla displays a unique feature that solitary hook and opposite hooks alternately occur in the leaf axils forming a liner series. The solitary hook only grows on the lower side of the leaf axil, resulting from the decline in the total yield of hooks. However, the reasons for solita...