Cotyledons of 11 species were studied at a number of stages of germination. The hypogeal pea and runner bean cotyledons did not expand, lost weight, and survived for a relatively short time only. They also produced little chlorophyll on exposure to light, possessed no stomata, and had a very low capacity for 14CO2 fixation. The epigeal french bean had cotyledons that were basically of the hypogeal type. Although both white and blue lupin cotyledons showed a progressive weight loss, they underwent limited expansion and were more persistent than either pea or bean. They also produced considerable amounts of chlorophyll, had stomata on both upper and lower surfaces, and fixed restricted quantities of 14CO2. The cotyledons of the other epigeal species studied showed varying degrees of expansion, up to almost fifty-fold in cucumber, and generally maintained or increased in total dry weight for at least a restricted period. Stomata occurred on both upper and lower surfaces, extensive chlorophyll production took place, and 14CO2 fixation values were high. Expansion was determined by increase in cell size, and not in cell number except in the case of cucumber where both factors were involved. In species where cotyledon cells were large initially little or no expansion occurred, whereas initial cell size was small in cotyledons which expanded to a large extent. Epigeal cotyledons with a high expansion factor possessed other characteristics which made them adapted for photosynthesis, whereas epigeal species with lower cotyledon expansion together with hypogeal species were less well adapted. This was not unexpected in the case of pea and runner bean, but led to the conclusion that french bean cotyledons are ‘accidentally epigeal’ in that they showed virtually no adaptation to an aerial existence. The different capacities of the cotyledons studied suggests that they have differing roles in the control of seedling growth.