Explanation:

The meristem which is responsible for the increase in the length of roots is called apical meristem. 

Apical meristem is a meristem at the tip of the plant shoot or root that produces auxin and results in the shoot or root to increase in length. Growth which gets originates in the apical meristem is known as primary growth. Due to cambial activity, lateral meristem and cork cambium, the width of a plant will increase. Apical meristem is the undifferentiated meristem. Apical meristem is present at the tip of the shoot. This apical www.gradeup.co 47 meristem is responsible for the growth of the plant right from seedling stages. It is also present at the root tips. Intercalary meristems are present in stems at the base of nodes and leaf blades. Cells of such meristems can divide and form new cells. 

Lateral meristems are vascular cambium and cork cambium. They are responsible for the secondary growth of the plant. They increase plant diameter.