After elongation, plant cells begin to
die
photosynthesize
differentiate
germinate

Elongation is a developmental process which is regulated by light and phytohormone. After elongation, plant cells begin to differentiate.  During the period of differentiation cells of the root and the shoot apical meristems and the cambium mature, the cells cell evolve in to a specialized cells and begin to take on their individual functions.

genetic variation can be caused by mutation (which can create entirely new alleles in a population), random mating, random fertilization, and recombination between homologous chromosomes during meiosis (which reshuffles alleles within an organism's offspring).