Abstract:Xanthium italicum (Asteraceae) is an annual species bearing dimorphic seeds. Each of its capitulum produces only one upper seed and one lower seed. It is productive, adaptable, and harmful to crops and environment. For a better understanding of X. italicum′s ecological adaptation, we studied its seed germination, its plant growth characteristics, and its ecological adaptability. The results showed that: (1) Lower seeds needed 13 to 15 days to reach a 50% germination, which was 5 to 7 days earlier than that of upper seeds. (2) In creasing amounts of leaf area at the interim developmental stage of the 1st to 7th leaves on the main stems of the plants of the lower seeds were significantly greater than that of the plants of the upper seeds. The plants of the lower seeds were notably higher than the plants of the upper seeds during the same period. On the contrary, increasing amounts of leaf area at the early or interim developmental stages of the 9th to 13th leaves on the main stems of the plants of the upper seeds were significantly greater than that of the plants of the lower seeds. No differences appeared during the developmental process of the 8th, 14th, and 15th leaves on the main stems between the two kinds of plants. (3) The number of primary branches and secondary branches produced by the plants of the lower seeds were significantly larger than those of the plants of the upper seeds, respectively. From this we concluded that the lower seeds have stronger abilities in survival and competition and contribute much more to the persistence, growth, and extension of the population than the upper seeds, that the upper seeds play an important role in keeping a stable population, and that to bear dimorphic seeds is an effective adaptation strategy for X. italicum.