Origin of Lateral Roots
The origin of lateral roots is endogenous - from a deeper layers. The seat of its origin is pericycle where cells opposite the protoxylem divide and form a hump in the endodermis. The hump penetrates into the cortex, and emerges as a lateral branch.
Later, the hump differentiates into 3 regions of the root apex - dermatogen, periblem and plerome. Finally the lateral root comes out. The number of lateral roots corresponds to the number of xylem bundles.