The mechanisms by which plants modulate their growth rate in response to environmental and developmental conditions are unknown, but are presumed to involve specialized regions called meristems where cell division is concentrated. The possible role of cell division in influencing meristem activity and overall plant growth rate is controversial, with a prevailing view that cell division is secondary to higher order meristem controls. Here we show that a reduction in the length of the cell-cycle G1 phase and faster cell cycling occur when the rate of cell division in transgenic tobacco plants is increased by the plant D-type cyclin CycD2 (ref. 8). The plants have normal cell and meristem sizes, but elevated overall growth rates, an increased rate of leaf initiation and accelerated development in all stages from seedling to maturity. We conclude that cell division is a principal determinant of meristem activity and overall growth rate, and propose that modulation of plant growth rate is achieved through regulation of G1.