In tomato seedlings the effects of high temperature in delaying the enlargement of the shoot apex and of increasing the number of leaves produced before flowering, were counteracted by removal of the first two leaves during the early vegetative phase. In plants grown at 25° C. defoliation was followed by rapid enlargement of the apex and earlier flower initiation, the number of leaves produced before flowering being reduced to that in plants grown at 15° C. Defoliation of plants grown at 15° C. resulted in only a slight increase in the rate of apical enlargement, and the time to flowering and number of leaves were unaffected. Growth analysis showed that at the time of removal, the first two leaves were not self-supporting and at 25° C. were utilizing a much higher proportion of assimilate translocated from the cotyledons than at 15° C. These results are considered to support the view that at higher temperatures, the first two leaves compete strongly with the shoot apex for supplies of assimilate.