Studies on seedlings of Acer negundo, A. platanoides, A. pseudoplatanus, A. pseudoplatanus-atropurpureum and Fagus sylvatica showed that the smaller seeded Acers had a shorter lag period before the onset of rapid growth than the larger seeded Fagus. Relative growth rates and net assimilation rates were high during the early stages of seedling growth when cotyledons constituted the major proportion of the photosynthetic surface area indicating the importance of cotyledon photosynthetic activity. Acer negundo which had the lowest seed weight and the smallest initial cotyledon area of the four Acer species had the most rapid dry weight increase. It also had stomata on both upper and lower surfaces of the cotyledons in contrast to A. platanoides and A. pseudoplatanus which had stomata only on the lower surface. Although this difference in stomatal distribution could be an important factor in the higher photosynthetic rate and more substantial dry weight increases in A. negundo, it is suggested that the latter is due mainly to the more rapid rate of leaf production.