In owering plants, the primary root system is generated by the root apical meristem (RAM), a meristem that is formed during embryogenesis. e RAM will generate the seminal root (SR), which can branch to generate postembryonic lateral roots (LRs). ese LRs can branch to generate other LRs, which result in additional branching levels. Some plant species can also generate other postembryonic roots, called adventitious roots (ARs), from the stem. Because ARs generally develop from aerial axes, they are easily observable until they reach the ground. Paradoxically, their cellular origin and the mechanisms controlling their formation remain hidden and are much less known than those governing the genesis of underground LR. In most monocot species, ARs develop constitutively and constitute the main part of the brous root system. In dicots, ARs can also develop constitutively or in response to abiotic or biotic stresses conferring the plant some adaptive advantage (Barlow 1986). e induction of AR formation in response to biotic stress is illustrated by the induction of AR formation from any part of the plant by the pathogenic bacterium Agrobacterium rhizogenes through the transfer of genes from the bacterial genome that modify hormonal balance of the plant (Figure 9.1A). ese ARs are agravitropic, highly branched, and fast growing and synthesize compounds that sustain the bacteria. ese roots can be isolated and easily cultured in vitro because they do not require any hormonal supply to maintain their growth.