Alternanthera philoxeroides(Mart.) Griseb., a perennial terrestrial plant of the Amaranthaceae family, originates in South America, but has spread to many parts of the world and is considered an invasive species in the United States, Australia, New Zealand, Thailand and China. A. philoxeroides is highly tolerant to submergence and able to grow in wetlands, river and lake shores, and other submergence-prone habitats. It is found that under submergence conditions, A. philoxeroides usually develops diameter-widened internodes on its unsubmerged stem parts above water surface. The inundated tissues and organs of submerged plants usually suffer from hypoxia and energy crisis therefore. Carbohydrates transport in plants is an energy-consuming process, when a plant is partially submerged, the impeded carbohydrate transport in submerged plant tissues and organs due to energy deficiency caused by submergence may lead to carbohydrate accumulation in abovewater unsubmerged tissues and organs, thereby affecting the diameter growth of abovewater stem. To explore whether the diameter widening of abovewater stem parts of partially submerged A. philoxeroides correlates with carbohydrates accumulation in the stem parts, we carried out an experiment in which two treatments were applied to two groups of randomly selected A. philoxeroides plants, respectively, plants in the first group were partially submerged and the plants of the second group were set as unsubmerged control. It was shown by the results that: (1) The average diameter of internodes on the abovewater stem parts in partially submerged A. philoxeroides plants was larger than the average diameter of internodes on the belowwater stem parts, and the abovewater stem swelled up markedly compared with the belowwater stem. In contrast, the upper stem parts (counterpart of the abovewater stem parts of partially submerged plants) of unsubmerged control A. philoxeroides plants did not differ with the lower stem parts (counterpart of the belowwater stem parts of partially submerged plants) in internode diameter, and no swelling occurred on the upper stem parts. (2) The content of carbohydrates in the abovewater stem parts of partially submerged A. philoxeroides plants was significantly higher than that of the counterpart stem parts of unsubmerged plants. Our study demonstrated that partial submergence of A. philoxeroides did cause the hampered carbohydrate transport in the belowwater tissues and organs so that the carbohydrates accumulated in the abovewater stem parts and led to the girdling-like swelling of the abovewater stem.