These embryonic tissues in the mature plant body are called meristems. Meristematic cells are usually thin-walled, more isodiametric in shape than the cells of mature tissues, and relatively richer in protoplasm. Looking at a meristem under the microscope you can see an amazing variety of things happening. Many cells are dividing, these will look small and crunched together. Others are beginning their differentiation process and you can see the formation of secondary walls.
Differentiation is the process a cell undergoes to become specialized for a certain funciton.