What collenchyma is
Two tissues stiffen a plant. This is the first of them, and it is the one that does the job while the plant is still soft and still growing.
Collenchyma
Collenchyma tissue contributes rigidity and mechanical strength to the plant body. Its cells are modified parenchyma cells.
Modified how?
Take a parenchyma cell and change one thing about it — thicken the wall, but only at the corners — and you have a collenchyma cell. Everything else stays as it was.
Kept from parenchyma
Still living. Still has cytoplasm, a nucleus and a central vacuole. The wall is still made of cellulose.
Changed
The cells are elongated rather than round, and the wall is thickened at the corners. That is the whole modification.
Strength without giving up life
A cell can be stiffened only so far before the thickened wall shuts it off and it dies — which is exactly what happens to the next tissue. Collenchyma stops short of that. It thickens only the corners, keeps the rest of the wall thin, and so stays alive and able to stretch as the plant grows. That is why it is the support tissue of young stems.