Lateral Roots - Root Apex
Lateral roots normally arise at a definite distance behind the tip from areas close to or opposite the points of xylem star. So, a triarch root can have 3 rows of lateral roots and a tetrarch root four rows of lateral roots. It is interesting to look at the growth of lateral root primordia through the tissue of parent root. In gymnosperms and angiosperms the lateral roots are generally initiated in the pericycle. Primordia are formed by the periclinal and anticlinal divisions of groups of pericycle cells.
According to one view growing primordia partially 'digest' the tissues of cortex through which they pass. Another view point is that this process of penetration is essentially a mechanical one i.e. growing lateral roots essentially push through the tissues of the cortex. Relatively low concentrations of auxins and high concentrations of cytokinins near the tip inhibit lateral root formation (auxins are produced in shoot apex and are transported basipetally and cytokinins are synthesized in the root tip also). As the auxin concentration rises (as we go up) lateral root formation is induced.