In plant anatomy, which phloem components are made up of sclerenchymatous cells and are generally absent in primary phloem but present in secondary phloem?

Introduction / Context:
Vascular tissues in plants, namely xylem and phloem, are composed of several specialised cell types. Understanding which cells provide support, which conduct food, and which store materials is a central part of plant anatomy. This question focuses on the supporting elements within the phloem that are sclerenchymatous in nature and differ between primary and secondary phloem.

Given Data / Assumptions:

• The question is specifically about phloem tissue, not xylem.
• It mentions sclerenchymatous cells, which are thick walled and dead at maturity, providing mechanical support.
• These sclerenchymatous cells are usually absent in primary phloem but present in secondary phloem.
• Options include xylem fibres, xylem parenchyma, phloem parenchyma, phloem fibres, and companion cells.

Concept / Approach:
Phloem is composed of sieve tube elements or sieve cells, companion cells, phloem parenchyma, and phloem fibres. Phloem fibres, also called bast fibres, are sclerenchymatous and provide mechanical strength. In many plants, these fibres are poorly developed or absent in primary phloem but are common and abundant in secondary phloem produced by the vascular cambium. Xylem fibres are sclerenchymatous but belong to xylem, not phloem. Phloem parenchyma and companion cells are living cells and are not sclerenchymatous.

Step-by-Step Solution:
Step 1: Identify that the question is about supporting cells in phloem tissue. Step 2: Recall that sclerenchymatous cells are thick walled, usually dead, and provide strength. Step 3: In phloem, the sclerenchymatous elements are called phloem fibres or bast fibres. Step 4: These fibres are usually not prominent in primary phloem but are well developed in secondary phloem of dicot stems. Step 5: Xylem fibres are similar in mechanical role but are part of xylem, so option A does not fit. Step 6: Phloem parenchyma and companion cells are thin walled living cells and therefore not sclerenchymatous. Step 7: Thus, phloem fibres are the correct answer.

Verification / Alternative check:
Botany textbooks describe primary phloem as consisting mainly of sieve elements and companion cells, with very few or no fibres. Secondary phloem, which is produced by the vascular cambium during secondary growth, contains well developed phloem fibres that can form the basis of commercial fibres such as jute in some plant species. Anatomical cross sections of older dicot stems show caps or bands of sclerenchymatous phloem fibres on the outer side of the vascular bundles, confirming their location and nature.

Why Other Options Are Wrong:
Xylem fibres are indeed sclerenchymatous but belong to xylem tissue and not phloem, so they do not satisfy the condition of being phloem components. Xylem parenchyma is a living storage tissue within xylem and not sclerenchymatous. Phloem parenchyma consists of living thin walled cells involved in storage and lateral transport. Companion cells are living cells closely associated with sieve tubes and assist in transport of photosynthates. None of these except phloem fibres match the description given.

Common Pitfalls:
A frequent mistake is to choose xylem fibres because students remember that fibres are sclerenchymatous but forget that the question is about phloem. Another pitfall is confusing the general term fibre with any supporting cell without paying attention to whether it belongs to xylem or phloem. Carefully reading the tissue name and the description of primary versus secondary phloem helps avoid this error.

Final Answer:
The sclerenchymatous cells that are absent in primary phloem but present in secondary phloem are Phloem fibres.