Development of cotton (Gossypium hirsutum L.) squares is fundamental for flowering, fruiting and yield formation. However, little is known about the carbon translocated to developing floral buds from leaves and floral bracts. Studies were conducted under field and greenhouse conditions to characterize the morphology and physiology of square development, and to quantify carbon contribution of leaves and bracts to a floral bud. During square ontogeny, increases in area and dry weight (DW) of floral bracts, as well as the subtending sympodial leaf, followed a sigmoid growth curve with increasing square age. Net photosynthetic rate (P_n) of the sympodial leaf at the first fruiting branch position of main-stem node 10 reached a maximum when the subtended square developed into a white flower. Floral bracts had much lower P_n and higher dark respiration rate than the subtending sympodial leaf. Total nonstructural carbohydrate concentrations in the bracts were also lower than that in the leaves during square development. Bracts also had a higher sucrose fraction of total nonstructural carbohydrates than the leaves. The amount of ¹⁴CO₂ fixation by the bracts of a 20-day-old square was only 22% of the subtending leaf. However, at 6 hr after ¹⁴C feeding these sources, the bracts had supplied 56% of ¹⁴C-assimilate to the floral bud, and only 27% from the subtending leaf and 17% from the main-stem leaf. These results indicated that floral bracts might play an important role in the carbon supply of developing cotton squares because bract ¹⁴C-photoassimilates moving into the subtended floral buds was much more rapid than that of the leaves.