This paper introduces basic knowledge and morphological types of plaant stems,expounds that the modified stems of plants are essentially a special function produced by plants to adapt environmental change,which is stably hereditary,and points out that the key to the distinguishiableness of underground stems and underground roots is that stems have jounts,buds and leaf scars but roots do not have and that the key to the distinguishableness of modified stems and modified leaves on the ground lies in the differentitation of the sources and locations of the modified parts.