Two annual herbaceous species of Solanaceae namely, Datura stramonium L. and Capsicum frutescens L. were subjected to study the incidence of the secondary growth and consequents the formation of protective tissues in their roots. The main results were as follows : 1- At an early stage of growth, the xylem tissue at the base of the root accomplished a proportion of its final amount that was decidedly bigger than that needed for the shoot system already achieved by the plant. It appeared therefore that certain amount of water conducting elements should be developed prior to the successful start and continuation active vegetative growth. 2- The amount of secondary growth varied in roots of the two investigated species; it was clear that the bigger the amount the more readiness of the root to form a periderm-like structure until a true periderm was developed with the biggest amount. 3- The periderm-like structure appeared in the exodermis or the underlying cortical layers according to the species, while the true periderm developed normally in the pericycle. 4- The endodermis and pericycle may behave alike or unlike as to their response to secondary growth.