In the roots of vascular plants, the protoxylem lies towards the periphery and the metaxylem lies towards the centre. This type of primary xylem arrangement is correctly called what?

Introduction / Context:
In plant anatomy, the arrangement of primary xylem within roots and stems is an important diagnostic feature. The terms exarch and endarch describe the relative positions of protoxylem and metaxylem. Exam questions often test whether you can recall which pattern occurs in roots and which in stems. This question focuses specifically on the arrangement where protoxylem is at the periphery and metaxylem is toward the centre in roots.

Given Data / Assumptions:

• The tissue described is primary xylem in roots.
• The protoxylem is towards the periphery and the metaxylem is towards the centre.
• Options include exarch and endarch, plus distractors referring to xylem fibres and xylem parenchyma.
• We assume standard textbook definitions of exarch and endarch patterns.

Concept / Approach:
In an exarch pattern, the protoxylem lies toward the outside and the metaxylem lies toward the inside. This arrangement is characteristic of roots. In an endarch pattern, the protoxylem is found toward the centre and metaxylem toward the periphery, which is typical of stems. Xylem fibres and xylem parenchyma are individual cell types within xylem, not names for the overall arrangement of primary xylem. Therefore, the arrangement described in the question, found in roots, is called exarch.

Step-by-Step Solution:
Step 1: Focus on the spatial relationship described: protoxylem near the periphery and metaxylem near the centre in roots.Step 2: Recall the definition of exarch, where the first formed xylem (protoxylem) is located toward the outside, and later formed xylem (metaxylem) lies toward the inside.Step 3: Recall the definition of endarch, where protoxylem is toward the inside and metaxylem is toward the outside, a pattern that characterizes stems.Step 4: Eliminate options that refer only to xylem fibres or xylem parenchyma, as they do not describe the overall spatial pattern of primary xylem.Step 5: Conclude that exarch arrangement of primary xylem is the correct term for roots.

Verification / Alternative check:
To verify, visualize a transverse section of a typical dicot root shown in botany diagrams. The xylem is usually star shaped with arms pointing outward, where protoxylem is seen near the tips of these arms toward the periphery, and metaxylem is closer to the centre. These diagrams are commonly labeled as exarch. By contrast, in young stems, textbooks depict endarch xylem with protoxylem inside and metaxylem outside. This comparison confirms that roots have exarch arrangement.

Why Other Options Are Wrong:
Option A is wrong because xylem fibres are supporting cells and do not name the pattern of primary xylem. Option B is wrong because xylem parenchyma refers to storage and living cells within xylem and again does not describe an arrangement pattern. Option D, endarch arrangement, is wrong because this applies to stems where protoxylem is toward the centre and metaxylem outward, opposite to the root pattern described in the question.

Common Pitfalls:
A common mistake is to confuse exarch and endarch because the names can sound similar if you are in a hurry. A useful mnemonic is to associate exarch with external protoxylem and endarch with internal protoxylem. You can also link exarch with roots by remembering that roots explore the external soil environment, helping you to recall that exarch roots have protoxylem near the outside. This type of association makes it easier to avoid errors in exams.

Final Answer:
Exarch arrangement of primary xylem.