Concept:
The root apex consists of different regions arranged in a definite sequence. Each region performs a specific role in root growth and development.
The sequence from the upper portion toward the tip is:
\[
\text{Maturation Region}
\rightarrow
\text{Elongation Region}
\rightarrow
\text{Meristematic Region}
\rightarrow
\text{Root Cap}
\]
Step 1: Region of Maturation
Cells become fully differentiated and root hairs develop in this region.
Step 2: Region of Elongation
Cells rapidly increase in size, causing root growth in length.
Step 3: Region of Meristematic Activity
Cells actively divide through mitosis.
Step 4: Root Cap
The root cap lies at the extreme tip and protects the delicate meristematic tissue from mechanical injury as the root penetrates the soil.
Thus, the missing structure after the meristematic region is the root cap.
\[
\boxed{\text{(A) Root cap}}
\]
