Biological characteristics of flowers and examination of pollen viability at different developmental stages of Epimedium sagittatum (Sieb. et Zucc.) Maxim oleh Jiang-long He, Li-xin Pei, Bao-yu Ji, Hai-bo Wang, Hua Zhong, Cheng-ming Dong, Sui-qing Chen, Xiu-qing Li, Pan-pan Li