Abstract: The phytomer, consisting of a node, leaflike organ, internode, and lateral meristem in the axil of a leaf, is a basic structural unit of shoot growth and development. With scanning electron microscopy (SEM), the developmental characteristics of phytomers in alfalfa variety ‘WL168’ were observed throughout the developmental phases of the apical meristem in crown buds from sprouting to flowering. During the development of phytomers, the primordial compound leaf emerged in an acropetal pattern. Both the primordial axillary bud and compound leaf originated from the apical meristem, but they differed in the rhythm of development; phytomers showed two patterns of development. Pattern-I was featured by the faster development of primordial compound leaves than primordial axillary buds which appeared after the formation of lateral primordial leaves. For patternⅡ, primordial axillary buds grew faster than primordial compound leaves, initiating after the formation of primordial stipules. Before the crown bud emerged from the soil, the phytomer developed through pattern-Ⅰ, and immediately after emergence, through pattern-Ⅱ. When the number of aboveground phytomer reached 16-17, the axillary meristem became reproductive. It was concluded that the shift of phytomer development from pattern Ⅰ to pattern Ⅱ can be an indicator of the transition from vegetative to reproductive growth in alfalfa.