SummarySugar levels in the various species of the Saccharum complex suggest an evolutionary increase in sugar content. Under suitable ecological conditions, survival through sucker growth may be dependent on rapid mobilization of stored carbohydrate. Selection pressure for sucrose storage would then oocur if sucrose was more readily remobilized than other storage carbohydrates. It is suggested that this ecological situation occurred in New Guinea, and that natural and not human selection lead to the evolution of S. oJJicinarum.The relationships between total sugars, moisture content, and fibre content indicate that there is an apparent physiological limit to sugar accumulation in the genus Saccharum at about 27% of the fresh weight.

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SummaryThe efficiency of photosynthetic utilization of light in Calvin cycle plants was lower than that found in plants with the C4‐dicarboxylic acid path of CO2 fixation when measured in air. Inhibition of photorespiration by O2 depletion indicated that potential photosynthetic efficiencies of both groups were similar. The ability of C4 plants to utilize higher light intensities than Calvin cycle plants was associated with a greater affinity for CO2 at the carboxylation site.Photorespiration was detected in a leaf attached to an 18 month old sugarcane plant. The leaf was significantly less efficient than similar leaves from a young plant and displayed a greater resistance to CO2 transport. An explanation is advanced in terms of end‐product feedback from assimilate stored in the stalk.