Meristematic tissue responsible for increase in girth of tree trunk is:

Lateral meristem are present along the lateral sides of stem and roots, and are responsible for lateral growth of trees. Intercalary meristem is associated with the growth in length in the middle position, causing internodal elongation. Phellogen also known as cork cambium is the meristematic plant tissue responsible for the formation of the periderm. Apical meristem is involved in the growth region in plants found within the root tips and the tips of the new shoots and leaves. Related Theory When plant breeders want to re-generate plants that are virus-free using asexual multiplication (in order to obtain clones of the primary unhealthy plant), they often collect the apical meristem from the infected plant, and cultivate it in appropriate media, so that a new, complete and virus-free plantlets develop. High metabolic processes (Enzyme activity); High concentration of auxin and Cytokinin hormone (Low pH); Cell multiplication very high and plasmodesmata size very small which prevent the movement of the virus from one cell to another cell.