a. Cross-section through a young stem. The vascular bundles are separated. The interfascicular cambium is not yet developed. Between periphery and the ring of vascular bundles a collenchyma ring of several cell layers thickness exists. Compare the different thicknesses of collenchyma cells (and the vascular bundle cells) with the central parenchyma cells. b Cross-section through a one year old twig. Xylem elements become more and more important for the strengthening. The number of vascular elements is higher than in a., forming a nearly complete vascular cylinder that is just here and there interrupted by parenchyma cells (pith rays). The collenchyma has become unimportant. (R. DUDEN, 1985)