In plant anatomy, when in a stem the protoxylem is located towards the centre and the metaxylem is located towards the periphery of the organ, this type of primary xylem arrangement is called what?

Introduction / Context:
In botany, the arrangement of primary xylem within a vascular bundle is described using specific terms such as exarch and endarch. These arrangements give clues about the development of xylem and the type of organ, such as root or stem. This question asks you to identify the correct term for a stem where protoxylem lies towards the centre and metaxylem lies towards the periphery, a classic concept in plant anatomy.

Given Data / Assumptions:

• The organ is a stem, not a root.
• Protoxylem is located towards the centre of the organ.
• Metaxylem is located towards the periphery of the organ.
• The options include cell types and two arrangement terms: exarch and endarch.

Concept / Approach:
Primary xylem differentiation patterns are described as exarch, endarch, mesarch, or centrarch. In an exarch condition, protoxylem is at the periphery and metaxylem towards the centre, typical of roots. In an endarch condition, protoxylem is on the inner side and metaxylem towards the outer side, typical of stems. Xylem fibres and xylem parenchyma are tissue types, not arrangement patterns. The approach is to match the positional description of protoxylem and metaxylem with the correct term, which is endarch for stems.

Step-by-Step Solution:
Step 1: Recall that protoxylem is the first formed part of primary xylem and functions when the organ is still elongating. Step 2: Metaxylem forms later and has wider vessels and tracheids suited for mature conduction. Step 3: In stems, the usual arrangement is that protoxylem is nearer the centre (inner side) and metaxylem is nearer the periphery (outer side). This pattern is called endarch. Step 4: In roots, the opposite pattern is usually found, with protoxylem at the periphery and metaxylem towards the centre, known as exarch. Step 5: Xylem fibres and xylem parenchyma are components of xylem tissue and do not describe the arrangement pattern. Step 6: Given that the description in the question matches the stem type with inner protoxylem and outer metaxylem, the correct term is endarch.

Verification / Alternative check:
Plant anatomy texts often show diagrams of root and stem cross sections, clearly labelling xylem maturation patterns. These diagrams emphasise that exarch xylem is characteristic of roots and endarch xylem of stems. The text descriptions usually define endarch as "protoxylem towards the centre and metaxylem towards the periphery". This matches the wording of the question exactly and confirms that endarch is the correct answer.

Why Other Options Are Wrong:

• Xylem fibres: A cell type providing mechanical support; does not refer to arrangement of primary xylem as a whole.
• Xylem parenchyma: Living parenchyma cells within xylem, involved in storage and lateral transport, again not an arrangement term.
• Exarch: Describes the opposite pattern (protoxylem at periphery and metaxylem at centre), typical of roots, not the stem pattern given.

Common Pitfalls:
Students often mix up exarch and endarch because the terms sound similar. A useful memory aid is to associate exarch with "exit" or "outer" because in exarch xylem, protoxylem is towards the outside. Endarch can be linked with "inner", as protoxylem is on the inner side. Another mistake is to be distracted by names of xylem cell types instead of focusing on the spatial arrangement. Always read the description carefully and remember that in stems, endarch is the normal condition.

Final Answer:
This type of primary xylem arrangement in stems is called endarch.