Thickened at the corners

One feature identifies collenchyma under a microscope and nothing else in a plant looks like it: the wall is thick only where three cells meet.

Figure to be added

Figure 1.8 — collenchyma tissue in two views: a cross-section showing rounded cells whose walls bulge inward at every corner, leaving the flat parts of the wall thin, with the cytoplasm and nucleus visible inside; and a longitudinal section showing the same cells as long narrow columns.

The characteristics

They are living cells.
Cytoplasm, a nucleus and a central vacuole are present in the cells.
They are generally elongated cells, and in cross-section they take a polygonal shape.
The corners of the cell walls are thickened with cellulose, so the cell walls are unevenly thickened.
Intercellular spaces may or may not be present.

Why thickening the corners works

Where three cells meet, three walls come together at an angle. Filling that angle with extra cellulose braces the join, in the way a corner bracket stiffens a wooden frame. The result is a tissue that resists bending while every individual wall stays thin enough for water and dissolved substances to pass through.

Unevenly thickened is the phrase to write

Both collenchyma and sclerenchyma have thickened walls, and the single word that separates them is unevenly. Collenchyma thickens the corners; sclerenchyma thickens the wall evenly all the way round. Answering that collenchyma has a thick wall, and stopping there, describes both tissues and identifies neither.