How does primary growth occur at a terminal bud?

A mass of apical meristem lies just below the surface of the bud. Cells at the center of the mass are completely undifferentiated, and they divide continually during the growing season. The divisions push some of the cells toward the edge of the mass. Cells that are displaced away from the center of the meristem begin to differentiate. The shoot grows longer mainly because differentiating cells under the mass of meristem elongate as they take on their specialized functions. The apical meristem remains at the tip of the lengthening shoot, continually renewing itself and also producing cells that divide and differentiate below it.