APICAL MERISTEMS. “In contrast to most animals, plants have a tremendous plasticity in the form and function of their organs. This is particularly relevant in perennials, in which the apical meristem of at least one of their shoots remains indeterminate beyond its first phase of growth and development. Perennial plants organize their body growth in the vertical plane, based on the division and differentiation of meristems.”
http://onlinelibrary.wiley.com/doi/10.1111/j.1469-8137.2010.03360.x/full