In the development of primary xylem, the first formed primary xylem elements that mature early during organ elongation are called ___________.

Introduction / Context:
This plant anatomy question is about the differentiation of primary xylem in growing plant organs such as roots and stems. As these organs elongate, xylem elements mature in a particular sequence. Understanding the terms protoxylem and metaxylem, which describe early and later formed primary xylem, is fundamental for interpreting diagrams of vascular bundles and for answering questions about plant tissue organisation.

Given Data / Assumptions:
- The question refers to first formed primary xylem elements. - It implies early maturation during organ growth. - Options list protoxylem, metaxylem, and other xylem components. - Standard botanical definitions of primary xylem parts are assumed.

Concept / Approach:
Primary xylem develops from procambium during the primary growth of the plant. It differentiates into two main parts: protoxylem and metaxylem. Protoxylem is the first formed xylem and matures in young regions of the organ that are still elongating. It typically has smaller vessels or tracheids with annular or spiral thickening to withstand stretching. Metaxylem is formed later, when elongation is nearly complete, and has larger elements with more complex wall thickening. Xylem fibres and xylem parenchyma are additional elements of xylem tissue but are not synonymous with the earliest primary xylem. Therefore, the first formed primary xylem elements are collectively called protoxylem.

Step-by-Step Solution:
Step 1: Identify that the question asks about the earliest formed elements of primary xylem. Step 2: Recall that primary xylem is divided into protoxylem (early) and metaxylem (late). Step 3: Remember that protoxylem appears in regions of the root or stem that are still elongating and has smaller, more elastic conducting elements. Step 4: Understand that metaxylem forms later when elongation slows or stops and has larger, more rigid vessels. Step 5: Note that xylem fibres are supportive cells and xylem parenchyma are living parenchyma cells associated with storage and lateral transport. Step 6: Conclude that the first formed primary xylem is called protoxylem.

Verification / Alternative check:
In diagrams of developing vascular bundles, protoxylem is often shown near the centre in stems (endarch condition) or near the periphery in roots (exarch condition), indicating its earlier formation. Metaxylem is shown in the opposite position and forms later. Text descriptions clearly state that protoxylem is the first formed primary xylem, and metaxylem is later formed. These consistent definitions across plant anatomy chapters verify that protoxylem is the correct answer.

Why Other Options Are Wrong:
Metaxylem: This represents the later formed primary xylem that develops after protoxylem and has larger vessels, so it does not match the description of first formed elements. Xylem fibres: These are elongated supportive cells that add strength to the xylem but are not specifically identified as the earliest primary xylem. Xylem parenchyma: These are living parenchyma cells within xylem used for storage and lateral transport and are not defined by order of formation.

Common Pitfalls:
Students sometimes confuse protoxylem and metaxylem because both are parts of primary xylem and their names sound similar. Another pitfall is to focus on the terms fibres and parenchyma and assume that they represent stages rather than cell types. The key is to remember that proto means first, and meta often indicates later or change, so protoxylem is early and metaxylem is later. Linking the term protoxylem with early elongating regions helps avoid confusion.

Final Answer:
The first formed primary xylem elements are called Protoxylem.