Despite large genotypical and phenotypical differences growing roots have some morphological features in common as shown schematically in Fig. 14.3. Attached to the root tip is the root cap which is a structural entity and its cells are continuously being renewed during root growth. The root cap not only protects the root meristem and facilitates penetration of the root through pores (by secretion of mucilage as 'lubricant'), it is also the site of perception of chemical and physical signals (Sievers and Hensel, 1991). Behind the root meristem is the elongation (extension) zone where the root cap-transmitted signals are rapidly transformed into differences in expansion rate of the cells and the root. Root elongation is correlated with stretching of the cell walls, particularly the outer tangential cell wall which, for example, in maize decreases in