An experiment was conducted to investigate plasticity of meristem allocation under intraspecific competition in three annual species ( Xanthium sibiricum Patrin., Datura stramonium L., and Fagopyrum esculentum Moench.) with different architectures (monopodial, false dichotomous, and sympodial branching, respectively). There were two density treatments for X. sibiricum and D. stramonium, with plant–plant distances of 7 and 14 cm, and five density treatments for F. esculentum, with plant–plant distances of 5, 10, 15, 20, and 25 cm. We recorded meristem fates (inactive, growth, and reproduction) at several developmental stages and at the final developmental stage. Allocation of meristems to reproduction in all three species could be explained by allometric growth (“apparent plasticity”), but meristem allocation patterns to the other functions differed among the species. Allocation of meristems to growth affects and is affected by plant architecture. Our results suggest that vegetative growth is more plastic than reproduction in annual plants.