Abstract:The whole growing period (from the formation of cotyledon primordia till ripe) of cotyledons of Nelumbo nucifera takes about 30 days, but can be varied in 3–5 days according to the varieties, temperature, light duration and its intensity during the blooming season. The observations by light and electron microscopy show that the feature and structure of mesophyllous cell are changed greatly during its development. The developing pattern of the cotyledons is similar to that of dicotyledonous plants. For comparitive analysis the authors divide the whole developing process into four developing phases: Ⅰ, the phase of cell division and organ formation; Ⅱ, the phase of cell vacuolation, elongation and swelling; Ⅲ, the phase of main physiological function in which the materials are largely synthesized and accumulated and Ⅳ, the phase of dehydration, contraction, maturation and dormancy. The development of mesophyllous cells in different part of cotyledons is not simultaneous and the duration of each phase is also different. In general, the developing order is from the base to the top and first the outlayer then the centre. On the 25–26th days after fertilization almost all mesophyllous cells are developing into maturation and dormancy by order. This is the first report about the developing pattern of cotyledons and the ultrastructural changes in mesophyllous cells of Netumbo nucifera.