Describe in detail the primary and secondary growth of the tissues of roots

Primary Growth 
The root system also has an apical meristem, known as the root apical meristem. This acts in much the same way as the shoot apical meristem, causing extension growth. The main difference is this growth goes down into the ground, and roots, not leaves and branches, come from the root apical meristem. 
Secondary Growth
The width of a plant, or its girth, is called secondary growth and it arises from the lateral meristems in stems and roots. As with apical meristems, lateral meristems are regions of high cell division activity. However, the cells they make grow outward rather than upward or downward. Eudicots use lateral meristems to add to their width; monocots, however, do not experience secondary growth.