In the development of primary xylem, the later (second) formed primary xylem elements that differentiate after organ elongation are called _____________.

Introduction / Context:
This plant anatomy question continues the theme of xylem differentiation. After the earliest formed primary xylem elements (protoxylem) appear, additional xylem elements form when the organ has completed most of its elongation. These later formed elements are structurally different and are given a specific name. Understanding the distinction between protoxylem and metaxylem is important when interpreting root and stem cross sections in botany.

Given Data / Assumptions:
- The question refers to later or second formed primary xylem elements. - It implies that these form after the initial stages of elongation. - Options include protoxylem, metaxylem, and other xylem components. - The definitions of protoxylem and metaxylem are assumed from standard textbooks.

Concept / Approach:
Primary xylem is divided into protoxylem and metaxylem based on the timing of development and structural features. Protoxylem is the first formed xylem that matures while the organ is still elongating. Metaxylem develops later, when elongation is nearly complete, and its elements are usually larger in diameter with more elaborate secondary wall thickening, such as pitted vessels. This sequence ensures that the plant maintains water conduction as tissues stretch and mature. Xylem parenchyma and xylem fibres are supportive or storage components and are not defined by early or late formation. Therefore, the second formed primary xylem elements are called metaxylem.

Step-by-Step Solution:
Step 1: Identify that the question asks about the second formed or later formed primary xylem elements. Step 2: Recall that protoxylem is the first formed primary xylem, active in regions undergoing elongation. Step 3: Remember that metaxylem appears after protoxylem, once the organ is more mature and elongation slows. Step 4: Understand that metaxylem vessels and tracheids are larger in diameter and have stronger secondary walls than those of protoxylem. Step 5: Recognise that xylem parenchyma and xylem fibres refer to cell types within xylem rather than to developmental stages of primary xylem. Step 6: Conclude that the later formed primary xylem elements are called metaxylem.

Verification / Alternative check:
In diagrams of roots and stems, labels often show protoxylem at one pole and metaxylem at the opposite side, depending on whether the arrangement is exarch or endarch. Explanations clearly state that protoxylem is early and metaxylem is late. When describing maturation, textbooks mention that metaxylem matures after organ elongation is mostly complete. This repeated definition across sources confirms that the second formed primary xylem is metaxylem.

Why Other Options Are Wrong:
Protoxylem: This is the first formed primary xylem, so it does not match the description of later formed elements. Xylem parenchyma: These are living parenchyma cells in xylem involved in storage and lateral transport, not a specific early or late developmental stage. Xylem fibres: These are supportive cells that provide mechanical strength and are not defined as the later formed primary xylem elements.

Common Pitfalls:
Students sometimes memorise protoxylem and metaxylem but mix up which is early and which is late. The word meta suggests change or later stage, which can help you remember that metaxylem comes after protoxylem. Another pitfall is focusing on individual cell types like fibres or parenchyma instead of the overall developmental sequence of xylem. Keeping the simple association proto equals first and meta equals later helps avoid mistakes.

Final Answer:
The later formed primary xylem elements are called Metaxylem.