Answer: Stems and roots become longer during periods of primary growth at the apical meristem.
Step-by-step explanation:
Apical meristem or growing tips is the undifferentiated meristematic tissues that is found within the growing tips of root and new shoots or bud. It is one of the three types of meristem which are Lateral, intercalary and apical. It is called apical because it is found at the tip of roots and shoots. It main function is to cause growth of new cells of young seedlings at the root and shoot tips. During the periods of growth, the stem and the root become longer or lengthens due to new cells developing and growing in young seedlings.