In plant biology, the corners of collenchyma cells are characteristically thickened due to the deposition of which substances in their cell walls?

Introduction:
Plant tissues show different kinds of cell wall thickening patterns depending on their function. Collenchyma is a mechanical tissue that provides support to growing parts of plants such as young stems and petioles. A distinct feature of collenchyma is the thickening at the corners of its cells. This question checks whether you know exactly which materials are deposited in those corners to provide flexibility and support.

Given Data / Assumptions:
- The tissue mentioned is collenchyma, found in young stems, petioles and leaves. - The thickening is at the corners of the cells, not uniformly all over. - Several common wall materials are given as options: lignin, suberin, cellulose, pectin, cutin and chitin. - We assume a basic understanding of plant tissue types: parenchyma, collenchyma and sclerenchyma.

Concept / Approach:
Collenchyma is a simple living mechanical tissue that provides tensile strength while still allowing growth. Its cell walls are unevenly thickened at the corners. These thickenings are mainly due to the deposition of extra cellulose and pectin, sometimes with hemicelluloses. Unlike sclerenchyma, collenchyma is not lignified to a great extent. Suberin and cutin are associated with protective layers such as cork and cuticle, while chitin is found in fungal cell walls and arthropod exoskeletons, not in plant cell walls. Therefore, cellulose and pectin is the correct combination for collenchyma corner thickening.

Step-by-Step Solution:
Step 1: Recall the basic definition of collenchyma as a living mechanical tissue with unevenly thickened primary cell walls. Step 2: Remember that these thickenings are rich in cellulose and pectin, making the walls strong yet flexible. Step 3: Compare this to lignin, which is mainly associated with fully thickened secondary walls in sclerenchyma and xylem. Step 4: Recognise that suberin and cutin are waxy substances in cork and cuticle, respectively, and chitin occurs outside the plant kingdom. Step 5: Conclude that the correct answer is the combination cellulose and pectin.

Verification / Alternative check:
Standard botany textbooks describe collenchyma as having cell wall thickenings composed primarily of cellulose, hemicelluloses and pectin. Diagrams show the corners of cells with more intense staining due to pectocellulosic materials. Laboratory micrographs also confirm this pattern. Lignin staining is typically weak or absent in collenchyma compared to sclerenchyma, reinforcing that lignin is not the main thickening substance here.

Why Other Options Are Wrong:
Lignin and suberin: Lignin is typical of sclerenchyma and xylem, while suberin is common in cork; this combination does not describe collenchyma corners. Suberin and cutin: These form protective hydrophobic layers, not supportive pectocellulosic thickenings inside collenchyma walls. Chitin and lignin: Chitin is not present in plant cell walls, it belongs to fungi and arthropods; combining it with lignin is biologically incorrect for collenchyma.

Common Pitfalls:
Students often confuse different supporting tissues. Because sclerenchyma is heavily lignified, they may assume all supporting tissues use lignin. Another mistake is to mix up protective substances like cutin and suberin with structural wall materials. A useful memory tip is to associate collenchyma with flexible support in growing organs and to remember the term pectocellulosic thickening, which directly points to cellulose and pectin as the key components.

Final Answer:
The corners of collenchyma cells are thickened due to the deposition of Cellulose and pectin.