A valid simulation of crop growth and yield requires quantitative data on the effect of the environment on photosynthesis, respiration and organ development. Seedling growth and boll growth of cotton, (Gossypium hirsutum L.) were studied in the phytotron at Duke University. We found extremely high respiration rates in the seedlings and an efficiency in the conversion of photosynthate to dry matter of 33% for the first 0.3 g accumulated and 53.5% for the next 0.5 g. Boll growth is exponential at first and then becomes linear. Growth efficiency in young bolls ranges from 67% at 18 C to 77% at 28 C. Later boll growth efficiency was essentially constant at about 61% at mean temperatures commonly found in the cotton belt, e.g., 26 – 27 C. Photosynthesis and respiration were measured in field stands during the boll‐filling stage with 2m ✕ 2m plexiglass chambers. Light respiration varied with temperature (25 to 42 C) from 8 to 15% of gross photosynthesis at a light intensity of 700 Wm−2. The ratio of total respiration to photosynthesis (R/P) varied diurnally, ranging from 0.29 under intense light [0900 local standard time (LST)] to infinity at sundown.